In 2004, when I was a junior at CMU, I had the privilege of finding a typo in Donald Knuth's book Digital Typography. Of course, I was wrong about how to correct it, but Dr. Knuth sent me a reward check for the erratum anyway. Here's my email with Knuth's comments in pencil:
And here's the check:
In August 2012, I reported a number of bugs (namely, four) and one harmless redundancy in Knuth's CWEB port of "Adventure", for which I received this second check. Notice that Dr. Knuth has stopped distributing real banks' checks and now distributes reward checks drawn against the Bank of San Serriffe, possibly the only bank in the world that uses hexadecimal dollars.