Oh, and a personal anecdote - I had a good friend who was in a FWB relationship for a bit more than half a year... at least that's what he thought. She was going around telling people he was her boyfriend and even bitched some other girl out for getting too close to him. A mutual friend confronted her about it and she admitted that she had developed feelings for him and thought she could shift the relationship into something more serious, but it went terribly wrong. These were adult people, mid-twenties. It happens.