I once had a player that for some reason thought that Fireball (AD&D 2nd edition) was a utility spell.

Don't get me wrong, this guy loved blowing up the bad guys with it, but he also used it for everything else. If he had a chance to cast Fireball, he would.

At one point, the PCs were lost in a snowy wilderness, freezing to death. He cast a fireball overhead and warmed everyone for a bit. This lasted all of a few seconds and they were feeling cold again, so he cast it on the trees around them, starting a forest fire and giving everyone a new problem to deal with. At least they weren't freezing to death anymore.

He also used it to collapse tunnels and do other things like that, most of them fairly nuts. He once used it to dry a wet floor (Wet with acid, I believe). Yes, you read that correctly. Like I said, nuts.

I liked having that guy in the group. He was just so crazy and the other players were constantly yelling, “No!!!!” at him. It was way too funny.

At one point he intentionally cast a Lightning Bolt in close quarters with himself and a huge monster, so it would bounce back and forth enough times to kill the monster. Since he was being creative, I gave him a fair chance to get out of the way after the first few bounces. He got very banged up in the process. The monster got worse.

Let's just say he had an interesting approach to using his spells. Crazy, but interesting.