A 502 isn't a server unavailable error, it indicates a response containing invalid http.
I've hit this quite a few times in the evening, especially with Firefox. Less frequently with IE7, for some reason. The only think you can do from your end is clear your browser cache, which could help.
Otherwise, someone's got a misconfigured or corrupted gateway or server somewhere in the chain from the site to you.
I have not seen this issue let me know if it continues I will monitor this thread and if it is an on going problem I will alert the admins. This issue is not realy relater to the EVO so I am moving this thread to the General chat forum.