When you try to break up with your significant other (or actually do) but continue to sleep with them afterwards; normally for lack of a new sexual partner.
Is that Ashley in your bed mate? Well that was a total failcut.
failrunning is similar to freerunning (aka - parkour), can be equally athletic or extraordinarily painful, but is only impressive if done purposefully and skillfully.
It happens skillfully when done on purpose in the same way stuntmen fall and look hurt in fight scenes, or it can happen by chance when someone either has an accident, or is just incapable of performing an action.
I saw a failrunner fall down 3 flights of stairs and get up without a scratch.
Some guy tried to show off his parkour, and ended up failrunning off a balcony and crushing his nuts on a bike rack.