It is spelled differently from crackin, although still having the same meaning, because the original word has the letters "ck" standing for cripkiller.
When you are out somewhere and you have to take a crap really bad but you think you can make it home. While driving home you hit a bump and explode in your pants. This is a crapcident.
I was rushing to the toilet but did not make it and had a crapcident in my shorts.