Some time ago I transposed the first letters of Yankee and Whisky to produce a callsign that definately isn't in the R/T manual nor I've ever seen.
Also, having recently asked a helicopter if he was visual with his company colleague, when he had in fact just landed behind him, I'm in no position to take the piss.
Although there's a time and a place, sometimes we'll just find something amusing and it doesn't take much to get the giggle box going with some things which pilots, engineers or atco's say.
It's rare that we'll laugh at the individual, it's more often than not what they've said.