A Google AdSense Help thread reports that one publisher not only received his check in the mail this month, but also received the check for another publisher in the same envelope.
The publisher said in the thread:
My payment cheque arrived with someone else's cheque too. I don't in the least mind posting it on, but I feel I ought to let Google and the cheque's owner know what's happening. Who do I email?
Google emailed this publisher directly, to not drive more attention to the issue in a public setting.
I assume the automated check machine and mailer accidently had some paper glitch and one check got stuck to another and both were placed in the same envelope. This is the first time I heard of such an issue with Google AdSense before.
It is funny in that Michael Arrington received a $1.3 million check by accident the other day.
Forum discussion at Google AdSense Help.