I prefer to have as few bits of paper as possible with me so I sometimes write reference numbers in my diary.
When I quoted one to an officious Servisair check in agent at LGW he said he needed to see it in writing, I just thought he'd misheard me, so I handed my diary across to him so he could read it. He thought I was trying to be funny, in fact I wasn't, and said he meant he needed to see the print out. When I told him I didn't have one he puffed himself up like a fat little toad and told me he 'had the right' (I think he actually said : "I am empowered") to deny me boarding.
"Go ahead, this will be interesting" I said and stood there. 'Just this once ....' he let me through. I took it up with the Servisair senior agent on duty who said that he was 'possibly being excessively rigorous.'