I've got 5 monitors connected to 2 ATI cards (Linux + Xinerama).
The most interesting artefact I've seen is some apps can corrupt the cursor so the pointer is a little bit of random memory contents. But only on some monitors. Move it to another monitor and it may come back, move it to the original monitor and it dies again.
There must be some really fun bugs in their drivers that rear their heads with massive setups.
I actually get this exact same problem on my Windows 7 desktop (3 monitors). The primary display cursor will sometimes have fragments of the cursor graphics or loading animation displayed but moving the cursor across each screens fast and back again can sometimes resolve it. Interesting that its a problem on both platforms.