I have my global inbox email set to notify me of unread messages daily.
I just got an email notification message saying
The following items were added to your Stack Exchange global inbox since you last checked it on 2011-11-22:
The problem is, the message it's informing me about was posted yesterday, I read it yesterday (the red "you've got mail" indicator went away), and I've been on Stack Exchange sites several times since I got the message. Plus, it only lists one message, when there's another comment on the very same answer.
I've had this happen before, just a few days ago. In both situation, I'd read the message and cleared out the "new messages" indicator hours before I got the email.