I saw a game last weekend where during a punt the receiver (R1) signalled for a fair catch and muffed the ball, which was then picked up by one of his teammates (R2) and run back. My understanding of Rule 6-5-5 is that no member of R may advance the ball after a valid fair catch signal. Did the crew blow this call or am I misunderstanding the rule?

Sounds like they blew it to me.

Think about the intent of 6-5-5. Once a fair catch signal has been given the defense can now relax because they know R will not be returning the kick.

Once the muff occurs either team can recover, but not advance. K could never advance the recovered kick and R gave up the right to advance the kick once a fair catch signal is given.

Let me add this note for officials who don't remember the old timing rule. Before 1997 if K wanted the clock to start on the snap, they had to signal and make a fair catch. The fact that A1 signaled a fair catch and A2 caught the ball was not and, even today is not, a fair catch. Previous to 1997, if any other player caught the ball the clock started on the ready. That had great implications for a late in the game kick when R needed the ball and the clock.

Ed Hickland missed one thing when he said K could never recover and advance the muffed punt. K can advance it if the kick is recovered behind the neutral zone. This is significant in that if they recover as such, advance and fumble R is now able to advance if they recover and the player who gave the signal may block. His prohibition to blocking ends when the kick ends which 99.99999999% of the time is also the end of the down.This is the only time that I know of that this prohibition isn't the end of the down but the end of the kick as specified in the rule book.