'An unusual No ball...'

"It's so unusual to hear an umpire at square leg call No ball that we spectators sat up with a start when it happened in a match last season. Yet there weren't more than two fielders behind the popping crease on the leg side, and it certainly didn't seem as if the bowler had thrown the ball. When they came off for tea I heard fielders talking about the wicket-keeper 'encroaching'. What did they mean by this?"

The umpire at striker's end called and signalled No ball because some part of the wicket-keeper's person or equipment was not completely behind the wicket when the ball came into play. The wicket-keeper must remain wholly behind 'his' wicket until either the striker touches the ball with his bat or person, or it passes the wicket, or the striker attempts a run

Read more about Law 27 (The wicket-keeper) at the MCC website