ZDoom

You must have had some really bad luck with your timing. This appears to be caused by alt-tabbing away right after the game sets the display mode but before it creates any surfaces. This seems to be really hard to handle cleanly, and I think I've got it fixed now.

However, I can also avoid the problem entirely. ZDoom was initializing the display twice at startup because I had commented out some code to avoid this. After uncommenting that block, I can immediately alt-tab away from ZDoom right after the screen goes black, and it will properly stay minimized and come back as expected. I don't remember when or why I commented it out, but hopefully that's also the reason for the two reports of ZDoom freezing at startup that I've seen here.

Edit: Just checked, and that code isn't commented out in 2.0.47, so that's probably the reason some people couldn't get it to start.

I started up v48, and as the console was loading, I switched away to answer an IM. As I started typing, ZDoom rudely switched back to 320x200(the default since I hadn't changed the resolution yet), and promptly crashed like a normal Windows app. But because I was in 320x200x8 at the time, I couldn't see a thing and the text was blacked out. I clicked where the 'x' would be and my screen went back to 1024x768x32 after ZDoom completely closed. But now all the standard Windows widget's colors are messed up(the the colors messing up sometimes happened with older versions too when I alt+tab away, so it may be a DX problem).

DX9 and Win2K. Latest video drivers, I think.