The act of taking a crap, finishing the roll of toilet paper, and neglecting to replace the empty roll with a new supply. This sets the stage for the next unsuspecting individual to sit down and pass a bowel movement not realizing there isn’t a square-to-spare (i.e. square of toilet paper, from Seinfeld). The ambush can easily be prevented by keeping a spare roll of toilet paper under the sink provided the dumb ass who is saved by the spare replaces it.
Oh Sweetie (i.e. significant other)! Someone forgot to replace the toilet paper again and I got ambushed!