I've only ever failed a spot once, never let it happen again.
Spotting my friend while he benched, don't know what happened but it happened quickly. Bar slipped out of his hands and bounced on his chest. It wasn't alot of weight but he's only a little guy and thankfully all it did was knock the wind out of him.
I was paying attention but it happened so fast i'm not sure i could even have done anything but i still felt bad.