There is an extremely large thread at Google AdSense Help group with publishers complaining that they have not received payment this month from Google.

Jennifer, a Google AdSense representative, confirmed that Google did not pay some publishers due to a bug of some sort. She said:

Jennifer makes it sound like this is just impacting YouTube partners, but I am not too sure about that.

Google is known to have made many mistakes paying publishers in the past.

Forum discussion at Google AdSense Help.