I've seen this before, and it's definitely just an issue with Safari caching the response served up by Gravatar. For some reason, and only on rare occasions, Gravatar seems to serve up something that Safari doesn't like and it caches that response. Why? I don't know. I guess that Gravatar have a minor intermittent issue which means they serve up the wrong message (maybe related to their system load?) and Safari has a minor bug in its cache management. Two minor things in combination become an occasional frustration for a few people.
What this definitely isn't is a bug with StackOverflow (or any other SE site) as they just serve up the correct link inside an
<img> tag. Nor is it anything that they can do anything about.