It's when a person fails to do something or says something that just doesn't work. When the result of something is totally unsatisfactory and/or potentially embarrassing.
It's like when a cake tastes like a dry piece of cornbread with super sugary icing on top: basically unpalatable.
Person 1: Did you see that guy trip over his pant leg just now?
Person 2: Yeah, that was a total fail cake. Sucks to be him.
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.
FRIEND A: "Did you just take a stealthie of me?"
FRIEND B (turning phone around): "no I was just using snapchat's new filter, see?"