Hilarious Ways to mess with your players.

[supermonkeyjoe]

As soon as someone gets see invisibility or similar endless fun can be had.

Villain turns invisible in a graveyard, you can now see him, and the legions of restless spirits wandering the area, better hope they don't figure out that you can now see them!

You cast see invisibility? You see that phantom fungus, and also that guy you killed back when you were second level. He's been following you ever since, watching everything you've been doing

You cast see invisibility, you can now see the invisible medusa.