The inconsistent, unpredictable quality of the shitting experience at roadside gas stations and highway rest stops.
Every person will encounter a broad spectrum in their lifetime, from the sparkling clean place that smells good, to the dark, musty smelling place with a pair of shitty underwear on the floor, a few gay sex propositions scribbled on the wall and a paper towel dispenser that hasn't been refilled since the Reagan administration.
The outdoor appearance of the building doesn't always reflect what the restroom will look like. Even if a bathroom has a good reputation, that can all change with one truck driver in a matter of seconds. It's 100% unpredictable, it's shit-or-miss.
Rod: "Okay, take this next exit, I am about to shit my pants."
Richard: "Okay Rod sure thing."
Rod: "Hurry up Richard, my stomach is really bubblin'."
Richard: "Okay Rod, what'll it be, the Shell station, or the Texaco?"
Rod: "Which one looks the cleanest?"
Richard: "The Shell station does, but you never know Rod, situations like this are shit-or-miss."
Rod: "Just hurry up asshole."