He said, "It's veal."
Almost without a pause, I said, "Okay. I'll have a cheese and pickle sandwich, please."
My colleague roared laughing, nudged me, said, "Classic!" and the guy behind the counter was all annoyed.
So one thought I was trying to amuse him - a bit of the Two Ronnies, maybe - and the other thought I was trying to offend him, but in fact it was only me mentally processing that I didn't want veal, and thought I'd stick with cheese, but without revealing it to my audience. I remember feeling a bit embarrassed, and, once I'd seen that I'd annoyed him, apologising to the man behind the counter. I'd not yet worked in catering...but somehow knew not to offend anybody in a place I went to regularly!