Enemy invisibility.

[maniacalmojo]

If your group uses a graph and miniatures during battling and such as i assume most groups do..

How as DM do you keep track of your own monsters or NPC when they turn invisible without making it REALLY obvious to players where the monsters move too.

[Perturbulent]

I simply pull them off the map at that point. I figure if they players can't trust me, whether or not they think I moved the piece is the least of my worries. Additionally, were I to have 30 invisible things or some other similarly annoying number to keep track of in my head, I could always draw a separate map for me. That way when they call out b10, I can shout: You sunk my Ogre Mage!

[Muggins]

If your group is using a graph and miniatures, it helps to have each axis coded. Alphabetical for one axis and numerical for the second gives you scores like B4, A9 and G3 to refer to. If you're got an invisible NPC running around that you need to keep track of, you can keep their coded location on a notepad behind your DM screen.