This is where the woman has forgotten that the cock was just up her butt and the man sticks it up to her mouth and she starts sucking on it. Only to remember instantly when she tastes her own pooh on his pecker. Then the woman usually instantly spits out some brownish looking liquid and says 'Yuck!' See also Sneaky Sanchez or Dirty Sanchez
After anal sex your girlfriend goes to the bathroom and is sitting on the toilet shitting out the wad you just dumped up her ass. You walk in and stick your dick in her face and she starts sucking on it. She tastes the shit and spits in the sink and you walk out giggling and very proud of yourself as she's calling you an asshole. Thus she becomes the Forgetful Sanchez.