Yeah, seems to happen a fair bit, its normaly a sign of either a dodgy connector on the c/l motor thingy....
Or the start of the relay in that particular mech wearing out! Happened on my drivers door... (stopped unlocking alltogether with the c/l). The doors have 2 locking mechs on them, 1 is dead lock, 1 is standard lock.... first button press releases the deadlocks - second releases the lock... if 1 of the relays screws up... the doors wont unlock on c/l but will normaly Lock.
hth