A meme (or other widely-shared photo or factoid) that is meant to be deliberately deceptive.
Deceptomeme purveyors prey on people's ignorance and the speed and zeal with which the uninformed will share unsubstantiated material.
People feel shame when you point out they have fallen victim to one of the classic blunders: passing along a deceptomeme.
"Hey, did you hear? This month has five Fridays, five Saturdays, five Sundays. That has not happened in 845 years. It's called Monkey's Luck and you are supposed to go buy a lottery ticket or some shit like that and then share it. I think I'm gonna do it even though this could be a deceptomeme."
"Check this out...tomorrow is the day Marty McFly set his time machine when he travels to the future. That is so cool because I really love that movie. I won't check to see if it's really true, but I'll just pass it along. No way this is a deceptomeme"