As told by Allison Falk...
I heard this story from a good friend of 11 years. It supposedly happened to her aunt's coworker. I believed it until I saw the same story circulating via email on two separate occasions (Hmm...). Here it is:
A woman is driving down the highway. The highway was originally in New York, but in the retellings I read both took place in Pennsylvania. The woman notices a police car behind her with his lights on and she starts to get nervous, planning what she will say when the officer pulls her over. The officer appoaches her vehicle and before he demands her license and registration, she chimes in, "I know! You pulled me over to invite me to the policeman's ball!"
To which he replies, "I'm a state trooper, ma'am. We don't have balls."
Upon realizing what he has said, he clears his throat, tips his hat and walks back to his car.
Comments: Compare the above to the following variant of the same "sex-word-error story" recorded over 20 years ago in The Choking Doberman by Jan Harold Brunvand (W.W. Norton, 1984), p. 139:
A bright and beautiful young lady zipping home for the weekend from college in her nifty sports car is stopped by a highway patrolman for speeding. Taking a chance, she says to the cop as he starts to write the ticket, "Couldn't I just buy a couple of tickets to the policeman's ball instead?" And the patrolman is supposed to have replied, "Miss, state policemen don't have balls!"
And to this version found circulating on the Internet in 2000:
A young woman was pulled over for speeding. As the motorcycle officer walked to her car window, flipping open his ticket book, she said, "I bet you are going to sell me a ticket to the Motorcycle Cop's Ball." He replied, "Motorcycle cops don't have balls." There was a moment of silence while she smiled, and he realized what he'd just said. He then closed his book, got back on his motorcycle and left. She was laughing too hard to start her car for several minutes.
The story is constructed like a joke and may well have found its way into popular culture via some comedian's standup routine, though nowadays it is always related as a true story that happened to a friend of a friend -- which makes it, of course, an urban legend.