When you have sex in the dark with a girl that thinks you're a different person but you aren't aware she mistook you until after the deed is done.
"How'd you lose your virginity?"
"Oh, I pulled a Casper The Friendly Ghost, once I realized she mistook me for someone else I got the hell out of there!"
Fogey/fogy /fougi/ sl. (early 18C+, orig. Scot) old-fashioned, stuck-in-the mud.
Person with old fashioned ideas which he is unwilling to change: Come to the disco and stop being such an old fogey!
You think me an old fogeyand an old tory, his thoughtful voice said. I saw three generations since O’Connel’s time. I remember the famine. Do you know that the orange lodges agitated for repeal of the union twenty years before O’Connel did or before the prelates of your communion denounced him as a demagogue? You fenians forget some things. (James Joyce, Ulysses. Penguin Books,1992. p. 38)