This issue was [raised on Meta StackOverflow earlier][1].

> We don't usually require sticky IPs (in particular once you're logged
> in it shouldn't be an issue), but if during the process of logging in
> your IP address changes, that's extremely fishy, and that's why we
> bail out.

So essentially this happens if/when your IP address changes during the login process and this is probably an issue at your ISP level.

  [1]: http://meta.stackoverflow.com/q/170875/164542