I sometimes get this error too when I switch resolutions (video_set) and it's worrying me.
I have more than enough free memory, my game doesn't use that much texture memory anyway (the memory for post processing stages is probably a big chunk of it). I only use TGA files or bmap_createblack images.

I had it all (deactivating render stages, switching resolutions, freeing memory and allocating textures for the new screen size) logged into acklog.txt and a console but can't seem to find any of those logs right now. Will report and come back here when I have more information to share.