1. When a pregnant woman lies on her back, and someone stomps on her pregnant stomach. The resulting stain left on the wall from the fetus splattering is known as a babystain.
when one is in someone elses house, and finds the condom stash and pokes a hole threw every condom with a needle, ultamately making the one using the condoms to have as many unwanted babies as he has girls in bed.
The repository of knowledge one retains but doesn't realize one has until such knowledge is necessary in a given situation (e.g. recalling the name/face of an old acquaintance/friend/girlfriend)
"I was walking to work and some guy asked me where I went to school; I thought he was a creep until my backbrain kicked in and I realized we lived on the same floor freshman year."
"I couldn't figure out why she slapped me until my backbrain kicked in and I remembered that I slept with her best friend a few years back."