Oh god - I hope I don't have to write a follow-up to this. There's probably a bunch of hidden other firewalls I don't handle but most often I've found 403 and 503 to be the most common.
Yeah, I can't believe how stupidly locked down some of these networks are.
I once had an employer said they needed a "whitelist" of websites we wanted to visit instead of a "blacklist" of ones we shouldn't. That was an interesting day...