The result of a man's Post-Masturbatory Reflection Period
being cut brutally short (by the arrival of other people to the scene). Usually a parent returning home from work. With an extremely limited timeframe the male often has to forsake the ritual wipe up and throw his clothes back on, regardless of the huge blob of semen on his midsection - leaving a great big emergency stain on his clothes.
Probably occurs in females too, I'd feel to weird to ask them.
Guy ~ "Woah, mum's home. Sorry pants, but this calls for an emergency stain"
Mum ~ "Why is there a naked lady on the computer?"
Me ~ "I genuinely assume this happens to everybody"