A client I worked for recently delivered a lot of its applications via Citrix. We tested it before deploying, and then once we'd deployed it we tried it out on various desktops; given that the vast majority of desktop PCs were identical and spanking new, we thought we'd cracked it.
Until, that was, we had one user call and say that their installation didn't work. We wandered over and sure enough, the windows just weren't appearing properly. (Citrix has a concept of "seamless windows" - instead of having a complete remote desktop, you just see the application windows on your own desktop, just as if the app were running locally).
Now, the machines to the left and right of this user's PC worked fine. They were identical PCs, each with two screens and the same version of everything. So we figured we'd simply work through whatever differences we found between the machines' settings. After checking a few items - user logins and whatnot - we thought we'd check the display settings - after all, the user with the problem had a custom picture on her desktop, and the other two didn't. We removed the picture, and it was still broken. Then we saw that her PC was set to 32-bit colour depth, and the other two to 24-bit. Switch the colour depth to 24-bit and ... voila! Everything sprang to life. Switch back to 32-bit, everything goes nasty again; back to 24-bit, works fine.
The users have now been "asked" not to change their display settings, and in due course the Active Directory global policy will lock the settings and prevent the user from fiddling. Thankfully our hunch that the problem was some weird, esoteric setting was correct, otherwise we could have been there for hours!
In the meantime: if you have a particular model of IBM/Lenovo desktop PC with two screens, don't try to be too colourful if you want to see your seamless Citrix windows.