When attempting to pass gas, you accidentally leave a "skid mark" (or worse).
The opposite of that old graffitti verse, "Here I sit, broken-hearted. Came to shit, but only farted." In this case you wanted to fart, but crap is what you got. This is especially true if you're over 50. Like Dad used to say, "Never trust a fart"!
"Did you fart?" "No, I think I SHARTED!"