most sky users will encounter this problem at some stage, the quickest way to sort the problem is to break the digital connection between the sky box and the tv or reciever best way simple switch it off normally a quick power down of the sky box will do it, if not kill power to all, this will reset the digital connection (handshaking) between sky box tv/reciever, but if this is a constant problem then i would suggest getting onto sky.
when you sometimes start playing a anytime stored programme you get out of sync problems but by pressing stop (not backup) then restarting also normally solevs the problem, happy hunting