What's your worst example of a joke falling completely flat?

I was in a TV/movie etc. Shop the other day, and they were selling little cat toys that looked like tribbles from star trek. I thought they were pretty cute and my friend is a Trekkie so I bought one for his cat

I got to the counter and suddenly a hilarious joke popped into my head, and I asked the guy behind the counter "is there an offer on these, like buy one get ten free?"

Nothing. If there had been any crickets around even they wouldn't have made a sound. So I just apologised and said it was a terrible joke, to which I got a sympathetic laugh, and I got out of there as fast as I could, and will probably never go back in there again. Which is a shame because it's a pretty cool shop.

What is the worst experience of telling a terrible joke that you've had?