A deceptive fart would be when you are about to fart, and just before the fart leaves your asscheeks, you realize it is in fact Shit. Therefore resulting in you having to clench said bumhole and try to suck the poo up. Can result in failure, where you shit yourself, or excruciating pain.
Robert: "Man I thought I had to fart but I shit myself."
Wesley: "That's the worst deceptive fart ever!"