@9, Lol man, service unavailable is not "oh, payments are shotty". Error 500 is internal server error, that's definitely out of the webmasters control. 503 just means that the server was unable to handle the request, i.e.: its being overloaded or is down for maintenance. The webmasters, again, have no control over this. Payments have nothing to do with it. Hell, it could be a cloudflare issue for all we know. The HTTP status codes that you need to be concerned with are:
410 Gone (I've never seen this one)
429 Too Many Requests
451 Unavailable For Legal Reasons
526 Invalid SSL Certificate
Or, of course, anything else related to the web host like if the domain has been parked, frozen and whatnot. If its hosted on a VPS, then 502 is also one you should be awear of. (This also goes for sites written in Ruby, Perl, etc.) However, I doubt we'll ever see 502 unless something really, really bad happens on cloudflares end.
Generally, if I'm operating a VPS, I keep track of all status codes under 4xx and 5xx, as all of those are error conditions of one kind or another.
"On two occasions I have been asked [by members of Parliament!]: 'Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out ?' I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question." — Charles Babbage.