Since this one doesn't fit the current challenge, I'll post it here. (It's the beginning of my fic-a-fest challenge, too.)
((No, I haven't forgotten the Halloween story. :/))
Harry had learned caution the hard way.
The first day, his shoe laces would not stay tied. He hadn't even realized it was a prank until he'd seen Ginny sniggering behind her hand.
The second day, his socks had gradually become colder until he'd been forced to go to Madam Pomfrey with frostbitten toes.
Every article of clothing, even his glasses, even his… well, everything… had been hexed at some point.
This morning, he cast a "Finite Incantatem" on everything he touched, even all the doorknobs.
When he was finished, he found that he couldn't let go of his wand.