Video drivers. Either update or reinstall. I've had this happen many times. To temporarily fix it, either reboot the PC or you can try changing the desktop from 32 bit to 16 bit and back or the other way if you are using 16 bit.
Since it has happened to me many more than 15 times with old drivers, I don't think your 1000000 % holds true . I never alt-tab in LFS. Not saying that your stated reason is wrong, just saying that you can't be 1000000% sure it is your reason since others have had this due to graphic drivers.
Have you tried a simple reboot? That often happens when the video memory becomes fragmented and the driver can't allocate contiguous blocks of memory to store textures in. It's fundamentally a driver issue of course since it shouldn't fragment memory like that, but a reboot at least lets the driver start anew.
I'm having the same problem lately ... not always but quite often.
My system is: P4 2,4 - 1gb ram- Ati 9800 pro 128 - Latest NGO drivers (1.73) ...
Rebooting system solves the problem , but is a waste of time !!! Any other solution ???