Has anyone ever booked an award ticket - no, this is not an airline-specific question - and then notice that the miles were not deducted out of your account?

A friend of mine just told me this happened to him, and even showed me a printout of his award-ticket-conformation! All of the flights seemed confirmed, he was able to access it on the airlines website, and everything.

Does this ever happen? What repercussions could he face?