AN: I'm in the process of re-watching the movies, and noticed that, after Ron's wand was broken, it was used exactly three times. Once, when Ron cast "eat slugs" at Malfoy, and, uh, had a taste of his own medicine so to speak. Then a second time, as Ron cast "vero verta", and turned Scabbers into a goblet, which still had quite a magnificent tail. Finally, it was used as Lockhart tried to obliviate the boys, and ended up wiping his own memory. So I thought, wouldn't it be interesting if his wand backfired every single time it was used, instead of just 2/3 of the time?
Disclaimer: I'm not even in college yet and I already eat ramen for dinner. Clearly I didn't write the books, and do not own the rights to them.
"You next, Mr. Wesley," Professor McGonagall said, giving a dubious look at Ron's wand.
Ron nodded, then tapped his wand three times before uttering the spell. However, the spell didn't uh, exactly go as planned.
Now this could be chalked up to several things. Maybe it was because Ron wasn't trying the spell on a true animal, and maybe it's trickier to transform a human into a water goblet. Or maybe it was because Ron didn't say the second vowel as precisely as Professor McGonagall had. But most likely, it was because of the large split in Ron's wand, only being held together by a few winds of spello-tape.
No matter the reason, when Ron cast the spell, the effect was rather unfortunate. Ron himself turned into a water goblet.
Let it be mentioned that Ron didn't exactly make a good water goblet. In fact, the water goblet had two feet with tattered shoes, a striped scarlet and gold handle, and a rather polished nose.
Of course, when one's classmate is unexpectedly turned into a water goblet (even if the goblet still had tufts of red hair and was emitting slight squeaking sounds), generally, the class no longer retains the ability to stay silent, even if their teacher were Professor McGonagall. As such, the class as a whole become a lot noisier, heralded by Hermione's shrill, chiding voice. Some students screamed, others talked excitedly, while others (not to point fingers, but largely those wearing green and silver ties) just laughed.
Above it all, Hermione could be heard saying "But, but, it was supposed to be Scabbers! Not Ron!" Eventually, she got over her shock and began throwing accusations at Professor McGonagall instead. Although it could be argued that since Hermione would never ever insult a teacher, Hermione was still very much in shock.
"But you said it would transform the animal! Not the caster! You said we wouldn't attempt any human transfigurations at all this year!"
"Miss Granger, do you really think that I would give my students a spell to become a water goblet? Now, can someone please run to the hospital wing for Madame Pomfrey?" Professor McGonagall said, simultaneously quieting the class and sending no less than four students running to the door.
In the end, Ron was able to become human again, although it did take the combined efforts of Professor McGonagall, Madame Pomfrey, and, to Ron's belated chagrin, Professor Snape, not to mention the better part of the afternoon.
Later, Ron said that it was just like when he had his head stuck in a too-small shirt (which was actually Ginny's, but that didn't really matter in the grand scheme of being trapped). He also reported being ravenous, and ate at least three sandwiches during dinner, as well as two chicken legs and a slice of cake.
There were a few benefits to Ron being trapped as an pathetic excuse of a water goblet for a few hours, one of which was it resulted in Ron getting a new and much more functional wand. Another was Ron was able to achieve a higher grade in potions than he had been, which confused Hermione to no end, as Snape (Professor Snape! Honestly, is it so hard to say "professor" first?) didn't even allow wands to be used in his class.
When Fred and George heard about the incident, and later received a detention from trying to use the spell to turn Colin Creevy into a water goblet (Hi Harry! No, don't worry, I agreed to it. Can I take a picture with you?), Ron was finally let off the hook, for stealing and driving a flying car to school, by his mother, as she was too busy sending howlers to the twins to bother scolding Ron. That was a third, and while unexpected, it was certainly not an unappreciated, benefit to spending a few hours as a water goblet.
Of course, since the tale of the incident spread through the school at alarming rates and actually became a spell used frequently between rambunctious Gryffindors and Slytherins for a few years after the incident, Professor McGonagall was forced to remove the spell from her second year curriculum, and instead only introduce it in her NEWT level classes.
And no matter how much Ron wished everyone would just forget the incident, he still, to ths day, hears the occasional "Hey! Goblet-boy!" from Dean and Seamus. Amusingly enough, one can even find mention of the incident in the latest edition of Hogwarts: A History, under the section labelled "Recent changes to NEWT classroom spells".