WARNING: THIS LIST CONTAINS PLOT SPOILERS!!! If you have not read the series, please look no further.
In the wonderful Harry Potter series by J.K. Rowling, the books are full of complex plots and sub-plots, which often interweave and connect as the series progresses, but that are very different in themes and tone. While much of the series focuses on the continuing struggle between good and evil, between Harry, his, friends, and teachers and Lord Voldemort and the Death Eaters, his menacing followers, they are also full of charming and often hilarious sub-plots involving whining ghosts that live in bathrooms, magic classes where the teachers know less than their students, spells and charms that accidentally backfire on their casters, and, of course, those darned magical creatures that Hagrid thinks are cute even as they chew on your fingers. In compiling this list I hope to have capture just a bit of the tonal changes in the series as they shift from humorous magical mishaps to tragic accidents to unexpected victories. So, without further adieu, here's my list of favorite magical mishaps, the moments that the Hogwarts folks wish they could forget about.
In Harry's first year at Hogwarts he learns why one should never waken a sleeping three-headed dog (even if it's name is something cute like Fluffy), why dragons don't make for good pets, why its more difficult to keep track of a key with wings on it, and the importance of playing wizard's chess.
Harry's first year at Hogwarts is indeed a memorable one in which he and the other first year students learn that as much fun as there is to be had with learning magic, there's just as much danger and nothing ever goes quite as you might expect. In addition to having some less than trust-worthy teachers and uncovering a plot to resurrect the evil Lord Voldemort with the Philosopher's Stone, Harry, Ron, and Hermione also find themselves face to face with a mountain troll that managed to mysteriously enter the castle.
The end result: While Hermione mopes and cries in the girls' lavatory a wandering mountain troll is let into the castle by nefarious means and it's up to Harry and Ron to save her. unfortunately neither of them are adept wizards and their mastery of casting spells is subject to debate... so Harry attempts to tackle the hulking troll and sticks his wand up the troll's nose leaving his friends to cast a spell causing the troll's club to knock its owner unconscious.
I do hope Harry will remember to properly disinfect his wand. Troll bogeys! Ew!
Who at some point or another didn't miss the school bus at least once and then had to figure out how to get there? Well, in the world of magic, if you miss the train to Hogwarts you'd better have a magical flying car ready... and you'd better know how to drive it.
In Harry's second year at Hogwarts, Harry and Ron are mysteriously prevented from entering Platform 9 3/4 and thus are unable to get onto the train to take them back to school, so Ron improvises and borrows his dad's magical muggle car, which just happens to be able to fit any number of passengers and it can fly. However, Ron isn't the most adept driver, and the car itself has a mind of its own...
The end result: Harry and Ron accidentally crash the car into the school's venerable Whomping Willow tree and receive detention. The car is somewhat banged up, but it survives the crash and drives off into the Dark Forest where it becomes a feral automobile, meanwhile Ron laments the fact that his wand was broken in the car crash, and Harry must deal with the conceited new Prof. Lockhart for whom he must sign autographs as a form of punishment.
And if all of that weren't bad enough, Hermione mistakenly used cat hair in a Poly Juice potion and turned herself into a feline-human hybrid and Ron's wand gets broken causing his spells to backfire on himself. Oops!
Who hasn't been given a textbook in school that was part of their required reading list and then realized that the book was giant volume that would take forever to finish? Well, in the world of magic, if you are given a required reading book in your Care of Magical Creatures class, you may want to have a muzzle ready... just in case the book tries to eat you.
Hagrid is lovable and loyal, no doubt, but he's also a bit of an oaf on a number of occasions. His judgment regarding wild beasts and magical creatures is sometimes lacking in discretion or awareness of danger. A good example of this can be found when Hagrid is given the chance to teach a class at Hogwarts and then decides that the Monster Book of Monsters would be a fun and educational learning tool. What was he thinking?!
The end result: The Monster Book of Monsters lives up to its name and as it turns out is just as unpredictable as the creatures in its pages. The book is basically a giant mouth that will devour everything it comes in contact with including other books or your shoes if you leave them on the floor. Ultimately, Harry and some of his friends were able to restrain their copies by binding them shut with their belts.
Also, a Hippogriff bites Draco Malfoy. Magical beasts should be treated with reverence and caution.
Sometimes a magical mishap seems to work to your advantage, while other times it leads to comical accidents, and still other times it leads to the forces of evil itself. When someone submits Harry's name into the Goblet of Fire and entering him to compete in the Tri-Wizard Tournament without his being aware of it, Harry finds himself overwhelmed and in over his head as he faces a series of obstacles that surpass his technical skills as a young wizard, but he still manages to stay alive and rise to the challenge.
He survives a challenge in which he must outsmart (and out-fly) a dragon in order to fetch a golden egg from its nest. Another task sees him swallowing gillyweed and growing flippers on his feet and gills in order to swim in the eerie lake where the merpeople have his friends magically suspended beneath the water. And finally, he navigates his way through a potentially deadly maze only to be transported by a portkey into a graveyard where he confronts the newly resurrected Voldemort, who is given his corporeal form via Black Magicks and the spilling of Harry's own blood.
The end result: While Harry won the tournament, a Hogwarts student is killed, Voldemort returns to power, and the history of the magic folk is forever altered.
On the upside, Hermione learns the importance of liberating house elves from their magical enslavement. Viva SPEW!
The fifth year at Hogwarts turns out to be even more dangerous and unforgettable than the previous ones as Harry and his friends must deal with the aftermath of Voldemort's resurrection and his rise to power. To make things even worse The Ministry of Magic has appointed a horrid new teacher by the name of Dolores Umbridge who abuses her power as a teacher and actually tortures students and enforces a rigid code of conformity. Naturally, there's little room for students to really learn or to have fun, so they rebel.
Professor Umbridge not only begins creating new rules by which the school becomes a place of militarized conformity and repression, but she also takes great pride in asserting that Harry is a liar. Not only is it revealed that she conspired against Harry, but she also punishes him by forcing him to repeatedly write "I must not tell lies" which literally cuts into Harry leaving a scar on the back of his hand. Things continue to escalate and Harry, Ron, and Hermione resort to create a secretive rebel group they dub Dumbledore's Army in order to prepare themselves for Voldemort's inevitable onslaught. Finally events climax when Umbridge discovers their group and an all-out revolt begins within the castle.
The end result: Fred and George Weasley fly through the great hall and unleash a set of magical pranks ranging from fireworks to dung bombs. And later Umbridge escorts Harry and Hermione into the Dark Forest where she is beset upon by an angry mob of centaurs who oppose her "Pure Blood" policies.
And that's not all, while storming into the Ministry of Magic on a rescue mission Harry and his pals accidentally knock of a shelf full of Time Turners causing a temporal loop for some Death Eaters.
In Harry's sixth and essentially his last year at Hogwarts, he experiences one of the greatest losses to ever hit the wizarding world and he learns much about the strange and disturbing past of Lord Voldemort. Headmaster Dumbledore and Harry begin making excursions into the Pensieve during the school year in which Dumbledore shows Harry memories of Voldemort's childhood and the events that lead to him becoming such a powerful Dark Wizard. Dumbledore also takes Harry on a series of missions to destroy the hidden horcruxes, items in which Voldemort has concealed parts of his soul so that he might live forever, but these don't necessarily go as planned.
During one of these missions, Harry and Prof. Dumbledore must take a boat across a haunted lake within an underground cave by the sea. There they discover that the only way to reach the horcrux is for Dumbledore to drink a potion held within a basin, which causes Dumbledore to experience maddening pain, and as he begs for mercy Harry must force him to keep drinking until he can safely reach the horcrux in the basin.
The end result: Harry and Dumbledore do manage to get the horcrux, just barely, but do so only to discover that it's actually a fake horcrux. Exhausted, both physically and emotionally, they arrive back at Hogwarts where the Death Eaters have made their attack during Dumbledore's absence.
Also, Snape's (partial) true identity is finally revealed!
After deciding that he could no longer attend Hogwarts without endangering his fellow students and knowing that he must continue the secret task of destroying the horcruxes, Harry sets out with Ron and Hermione leaving their beloved school behind. On their travels they not only come face to face with numerous dangers, but they also have time to reflect upon the nature of their friendships and exactly what it is that they are fighting to save.
While on the run, Harry and Ron are put at odds by the corrupting influence of a horcrux locket causing internal friction among the trio of friends which results in Ron storming off, leaving Harry and Hermione to complete their mission by themselves. Harry and Hermione return to Harry's birthplace in Gryffindor's Hallow and come across the tombstone marking Harry's parents graves. After visiting the cemetery they visit the home of Bathilda Bagshot where they hope to learn about Voldemort's past and perhaps information on the final horcruxes, but Bathilda isn't herself... literally! She turns out to be Voldemort's monstrous snake Nagini in disguise and she attacks Harry and Hermione who barely escape with their lives.
The end result: Harry and Hermione head off into the woods shaken and Harry later sees a patronus of a doe, which he believes may have been sent by his mother, which leads him to the Sword of Gryffindor hidden in a small lake in the forest outside of Gryffindor's Hallow. There he manages to retrieve the sword from the icy cold water, but the horcrux locket around his neck suddenly springs to life and tries to kill him. Then Ron unexpectedly emerges from the woods and saves Harry by using the sword to destroy the horcrux.
And of course, this being the final book, it concludes with the ultimate battle between good and evil as Harry and the teachers and students of Hogwarts take on the forces of Voldemort in their many forms.