Yes I've seen electrolytics go bad (luckily not on a mobo).
3 in total, first went brown and made the goop on a nearby LM317 boil from heat.
Second exploded and showered me with innards, major chunk hit the floor and burnt a hole in the carpet (flew right over my shoulder).
Third caught fire and burned for about 3 seconds with a yellow/white flame. Left massive scortch marks on the board.
Luckily in all three cases the surrounding circuitry was fine.the exploding cap actually fixed the short by blowing the leads apart
