Harry Potter and the Butterbeer on the Floor
Harry was walking to his common room, when he saw a Butterbeer machine. He was surprised, because he didn't know they had vending machines at Hogwarts. He bought a Butterbeer and continued back to his common room. On his way there, he spilled a drop on the floor without noticing.
Snape was patrolling the hallways. Suddenly, he saw something on the floor. He crouched down and sniffed it. It was butterbeer. This must mean there were students at Hogsmeade! Snape wanted to catch them. He left immediately.
Neville knocked on Snape's office door. He was supposed to have Remedial Potions with Snape, but Snape wasn't there. He told Professor McGonagall and she decided to teach him even though she was no expert. In the end, she ended up turning Neville into a cow, and didn't know how to change him back. She ran for help and ended up running into Hermione and Draco, making them fall over. She just continued running.
Hermione and Draco were secretly seeing each other even though Hermione was dating Ron. When McGonagall knocked them over, Hermione started to flip out and it took Malfoy a couple minutes to convince her that whoever it was that knocked them over, didn't see them and their secret is still safe. They continued on their walk, but didn't notice that Ginny, who was headed to the Quidditch pitch to practice, had just exited the washroom right behind them.
Ginny decided that if her brother's girlfriend was cheating on him, she'd have to tell him. She waited until lunchtime the next afternoon. She walked up to Ron and told him she was very sorry to have to tell him this, but Hermione and Draco were secretly seeing each other. Ron was very upset and angry, so he decided to put magical super glue on her favourite chair, making her stick to it and be late for Potions, which was right after lunch.
Dumbledore was headed to the Gryffindor tower to have a word with the Quidditch team. He walked into the common room and sat down on the chair that Ron had put the glue on. When he was done talking, he tried to get up, but couldn't. He called for help and a little second year brought him some scissors and he cut his robes in order to be able to get up. He ran out of the common room as fast as he could with his old, wrinkly butt hanging out from his robes. Nobody saw this except for Harry. He was scarred for life. He didn't know such a disgusting image could exist, and it lived vividly in his mind.
The moral of this story is, if you don't want to see your headmaster's wrinkly ass, don't spill your butterbeer.