I was once flying in the South East with a friend and we called Dunsfold radar who acknowledged us and blurted out the usual stream of info (QNH, squawk code etc.) We hurriedly wrote it all down, dialled the numbers in to the instruments etc. After a short delay the response came back from the controller....
'Golf Alpha Bravo you appear to be squawking the QNH'
Although the radio was silent I could hear the whole world laughing. Looking down at the transponder, sure enough it had 1013 dialled in! If only we'd tried getting 7890mb in to the altimeter we might have twigged!!