And I don't mean the mini-tower collapsed under my weight...
Years ago I had a very cheap case, a 586-133 (Cyrix, I think), and a 3Dfx Voodoo card... and there was absolutely no way I could screw the thing in - the card perilously gripped the PCI socket with all the tenacity it could muster. Which, since I went to a lot of LAN parties in those days, wasn't much.
Every move of the PC was accompanied by ten minutes of "can I seat the card properly?", often followed by "bugger it. PixelQuake (i.e. not GLQuake) will do..."