Have you ever been approached by a grim-looking man, carrying a naked sword with a blade about ten miles long in his hand, in the middle of the night, beneath the stars on the shores of Lake Michigan? If you have, seek professional help.
Harry, when he was home-invaded by a demon, tells Susan to go drink an escape potion in the basement. He later runs down to find she's still there, so he casts up a very small, circular barrier that will protect them, but will break if they cross it. The kicker comes when Harry realizes...that what Susan drank was a "Super Tequila" Love Potion. He then must try to control her, his hormones, and keep the barrier up. Hilarity Ensues.
This on top of the fact that he was just out of the shower, and buck naked.
At the end of the book, Harry is cornered in a kitchen by a horde of rapidly growing scorpions, one of which nearly killed him earlier in the book, with only a broom to defend himself. How does he survive? He casts a spell—one he came up with as a young apprentice to help with the housework—to make the broom sweep the scorpions away and off a balcony while they're still small. It's topped off with this line: "I'm pretty sure I swept up all the dirt too. When I do a spell, I do it right."