Using Spiral Knights in native res fullscreen, 1680x1050 (with vsync disabled to prevent the bad graphical "hitches") will regularly leave the SK graphics visible when using ALT-TAB to switch to another application such as a web browser. The cursor will change to the default Windows cursor, but the background graphics and suck for SK will remain visible and not allow the other application to be seen.

The system in question is:
Windows XP SP3
4GB RAM (3.5 available)
ATi 4890 (11.06 drivers)
Java JRE 1.6.0_26

This also occurred with older 10.12 catalyst drivers with JRE 1.5.0_22 (last version of 1.5 JRE).