A couple of weeks ago I picked up my MacBook and the R key felt squidgy, it was half stuck down and when I pressed it, it was like pushing dead flesh. I got straight on to Apple chat support and they booked it in for a repair. While I was chatting online to him, I managed to wiggle the key so it started working properly again. I still took it in though and they said they'd take it apart and see if they could see anything. I requested that they keep it and do a full, thorough check before I got it back so the guy just said they'd replace the entire top section.
Fast forward a week and I've picked it up with a new "top case with keyboard". I don't know if they've changed the design of the mechanism or not but if it happens again, I'll simply repeat the procedure.