International Institute of Cricket Umpiring & Scoring
Caught off Non-striker's helmet
"The bowler charged in and bowled a short ball, which took the batsman by surprise. He mis-hooked it and, instead of the ball flying past square leg to the boundary, it shot back up the pitch in the air. The non-striker moved to avoid the ball, but only succeeded in ducking into it. It hit him on his helmet and the rebound was caught by mid-on who appealed for the catch. But surely the ball was dead once it hit the helmet?"The ball is not dead if it hits a helmet, unless it's one not in use by a fielder - usually placed on the ground behind the wicket-keeper. If the ball hits a helmet being worn by a fielder a fair catch cannot be taken later but, in this instance, the ball was struck by the bat, hit the non-striker's helmet - not a fielder's one - and was then caught fairly.
Read more about Law 32.3 (Caught) at the MCC website