It is worth storing a diagram of the main status codes, conveniently. A good source is https://en.wikipedia.org/wiki/List_of_HTTP_status_codes
404s are fairly well known, but the 500 series, as indicated, reflect a server problem. Good web developers should also look at their logs, and if, for example, they receive regular hits from a link to a place that they no longer use, they should return "301" (moving permanently), which may cause the linking linker to update their link (or not).
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