We were a group of three, a guide and two clients. While climbing the North Buttress in reasonable winter condition, on the belay stance of pitch 3 the guide unclipped the lanyard of the other client thinking it was his own. Not until he set off did he realise the mistake, so for a short period the client was unprotected. The ledge was cramped and covered in stacked double ropes, the lanyards were clipped onto a threaded sling at low level, added to this the carabiners of the guide and client were the same make and model.