My callsign is GYBAA. it is sometime read back incorrectly, always as GBYAA. Presumably because there are very few UK aircraft with a GYxxx call sign, but many with GBxxx. The error rate is probably about 5 percent or so, and occurs both on 'freecalls' and 'contacts,' i.e. it doesn't seem to matter if the controller has pre-knowledge of the callsign or not (indeed the confusion may have arisen earlier on telephone handover, or on inputting the flightplan).
Also occasionally get called as something like GBYBB, but these things are never a problem in practice. It's slighltly comforting to know that controllers, as well as pilots, sometimes find their brain is not quite connected to their mouth when the pressure's on! :-)