I received an email in my priority inbox.  The contents of the email were replaced by a Gmail warning that said:

"Be careful with this message. The sender's account may be compromised, so this message could be a scam to steal personal information."

When I clicked on the "Learn More" link, the explanation said:

"Why am I seeing this warning?
The message was sent from an email address in your Gmail contacts, but the content and/or other attributes of the message look suspicious..."

The from address was definitely not in my Gmail contacts.  The message came from another email address and was pulled in by POP3.

I suspect there is a bug in the handling of phishing email from POP3 accounts that causes it to be treated as coming from a Gmail contact.