Oh, that really irritates me. I had something similar happen a few months ago; a local restaurant that we frequent fairly often has an ongoing deal with a radio station for discount gift certificates. I bought one in the lower denomination--all the more expensive ones were sold out.
I went a week or so later to use it on breakfast. When I tried to pay, the waitress (who had worked there for years, btw) looked at it, and said she didn't know how to use it, and the owner was out, and I'd have to pay a different way. I suggested she take it as currency and write my number on it in case there was a problem (small town, and the owner knows us on a casual basis.) No, no, she didn't know what these certificates were, I'd just have to pay another way. (I always kind of disliked eating there when this waitress was working, as she's rather pushy.) I paid another way, but went away feeling kind of annoyed. Yes, I would have eaten there anyway, and yes, there was no problem paying another way, but why have the certificates if the waitstaff isn't going to know what the heck they are? Plus, I felt really stupid during the transaction, you know? Not the way you want to be made to feel at a place you otherwise enjoyed!
I sent an e-mail to the owner when I got home, and explained what happened. I also noted that all the more expensive certificates had been sold out, and since I know people travel ~30-90 miles to try his place (it won an award last year), they might not appreciate being told their method of payment is bad. The owner was really apologetic and also dumbstruck as to why his waitress wouldn't take it. He said he'd talk to her about it, and I don't doubt he did.