I've had this a few times with Win7 64 bit and Chrome browser.
Remedy
Restart browser.
No problem here, refresh your browser contents (Ctrl-F5).
No problem here, refresh your browser contents (Ctrl-F5).
Yep, no problems here either.
(Chrome OS)
Oops, yeah shift reload fixed it. I didn't expect a 502 error message page to be persistent in cache.