A Google AdSense Help thread has some AdSense publishers acting concerned over noticing a message indicator showing one unread message but when they click through, there are no messages waiting for them.

One publisher wrote:

Some are very concerned, which boggles me since those messages are rarely important.

I personally do not see the issue and my messages read as zero but who knows how many are affected. There are a handful of reports about this.

Forum discussion at Google AdSense Help.