Even though curses can be played at any time, you'll have to honor the die roll before doing anything else.

Q. If you use a one-shot Item during combat and someone tries to make it disappear through Curse or Theft, do you get the bonus?
A. Theft doesn't work during combat. Use of a Curse could destroy an item as you try to use it, but once you play the item into the fight, it's not yours to be cursed anymore.

Q. Can I play Go Up a Level cards on another player – for instance, to make him go up to a level so that a monster that would previously ignore him will now chase him?
A. This is not the original intent of Go Up a Level cards, but it is such a munchkinly and vile idea that we like it too much to say no. So you can play Go Up a Level cards on another player. But just as when playing a Go Up a Level on yourself, the munchkin must legally be able to gain that level. You cannot play a Go Up a Level card, even one with a secondary effect, on a munchkin who has to kill a monster to gain that level.