In which match did the following happen? (I can't for the life of me find it.)
Djokovic hit a backhand hard crosscourt from inside the service box, approximately on the left tramline. Nadal was right on the baseline, guessed correctly which way Djokovic was going and hit a trademark Rafa forehand down the line for a winner (Djokovic was nowhere near, since he was in the left service box, but the forehand went deep down the opposite tramline).
However, the ball was called out on Rafa's baseline. Djokovic challenged, only for it to be shown to be in; Rafa was suitably annoyed, given that he'd hit a clear winner. The call didn't really affect Nadal either, since Nadal basically half-volleyed it. Djokovic, very sportingly, gifts the point to Nadal, saying that indeed Nadal hit the ball for a winner.
This is the most pointless challenge I've ever seen: challenge; be right; give away point. (Maybe Djokovic instinctively challenged? then realised Nadal had won the point anyway.)
I'm pretty sure it was French Open, maybe 2012 or 2013. But I can't find it!