I have not had this happen with a motherboard... But i have had other cases: Monitor that suddently started coming thick PINK smoke from, a printer that exploded with flame shooting straight up, pc speakers that exploded with thick gray smoke coming out of one of them.. The printer was worst, smelled AWFUL
edit:
oh yeah... forgot my seagate disk.. suddently went up in smoke :-(
Also got myself a nice hercules kyro board a few years back.... worked great for the first 10 hours i got it until i was going to use the tvout.. as i changed to dual view it worked for 10 seconds before the screen went black and board started smelling weird... never got it replaced, even though the board was new... :-(