Here's an issue that I stumbled across. Once I had a hunch what it was (tracing the last changes I'd made to the system), it turned out others had had the same problem before but it wasn't exactly common.
Sticky Keys, ye olde Windows keyboard accessibility feature that is handy when you are typing one-handed, for example, may make Windows Explorer go crazy after some time (weird flickering, taskbar jumping around wildly - very annoying). At this point, a check in task manager reveals that Explorer is using 9999 GDI objects, which is the maximum by default. The culprit, it turns out, is the tray icon for Stick Keys, which indicates the status of the keys it controls (Shift, Ctrl, Alt, Win). Each time it is changed, Explorer leaks a few GDI handles, rapidly accumulating them if you use these keys a lot (Shift being the most common candidate, but don't underestimate how often you Alt-Tab). Apparently this only happens when the icon is hidden, e.g. if you have an auto-hiding taskbar like yours truly. A bug, no doubt.
Workaround: Turn off the tray icon for Sticky Keys. Rapid GDI resource usage immediately stops.
This is in Vista. I am not sure which other versions of Windows are affected.
