The more you think about this one, the funnier it gets.
Short & sweet, a good one.

A very tired nurse walks into a bank, totally exhausted after an 18-hour shift. Preparing to write a cheque, she pulls a rectal thermometer out of her purse and tries to write with it. When she realizes her mistake, she looks at the flabbergasted clerk, and without missing a beat, she says:

“Well, that's great....that's just great..........some arsehole's got my pen!”