Originally Posted by Drip

Adding to that: Clear your browser cache after these maintenances! You'll save yourself a lot of non-existing bugs (they're not bugs, they're caused by the player using an outdated and not perfectly compatible version of the client against newer data on the server).
For Firefox, go to Tools - Options - Privacy & Security tab, then hit the Clear Now button.

That or you can use a handy tool like CCleaner to delete cache from Flash and any browser you're using in one go.