I'm guessing that this might have been mentioned already, but I couldn't find in the forum so I'll take my chances.
When changing from either windowed mode to fullscreen or vice versa, the graphics engine just dies and more or less everything in-game is white, rendering the game unplayable. A restart is the only solution I've found.
I've uploaded the debug0.txt here, in case that helps:
This is always reproducible on my computer (Hedgewars 0.9.11, XP SP3, NVIDIA 8800GT - 185.85 drivers) and on a friend's laptop (Vista x86, ATI Mobility Radeon - driver version unknown to me).