It was when you asked a simple question, or tried to be helpful, only to be snapped at by some ATCO that I used to get annoyed:
1. Transiting the Benson MATZ eastbound towards White Waltham, I asked for the London QNH. For obvious reasons. The response was "You should be flying on the Benson QFE!!". "7000, en route, good-day!".
2. Mil this time. Climbing up to FL100 for hi-rot spinning I advised Brize Radar that I'd be climbing from south to north, remaining above the CTR throughout. The response was "Not above FL80!". "Sorry, I
will be climbing to FL100 and this was for your information. 7000, to Stud 7, good-day!".
The best ever was when I said I was RTB and would like a radar-to-visual. "Controller isn't available", I was told. "OK, I'll just fly an NDB-to-ILS and call you when visual".
"I don't think I'm allowed to let you do that".
However, I'm sure things will have improved since those days....