That reminds me of my very first programming job, using a source control system (Rational ClearCase, I think) at a company that had licenses for a certain number of concurrent users. When you tried to check your code in, the client had to check out one of the license keys to use during the interaction with the SCM server. If they were all in use, the check-in would fail: "no license available, try again later."

I think that was a licensing scheme offered by Rational and not a clever hack we devised for screwing Rational out of license fees, but I wasn't sure and wasn't brave enough to ask.