I just got this error message for the first time (I was getting this as the response to any request on the SE network for about the last five minutes, until now):
There are an unusual number of requests coming from this IP address. To protect our users, we can't process any more requests from this IP address right now. We'll restore access as quickly as possible, so try again soon. If you believe you have reached this page in error, contact us.
As far as I know, I was not spamming SE or participating in DOS attack, and I find it unlikely that any of my office coworkers who share my IP address were doing so either.
Granted, I was pressing F5 a lot. But I've always pressed F5 a lot on SE, and have never gotten this message before. So my question is: Is it possible to get this error message simply by refreshing too much or too often? Or is that message reserved for a truly massive number of requests that are unlikely to be caused by one person refreshing a lot? (In the former case, I can't explain why I've never gotten it before; in the latter case, I can't explain why I just got it now).