[xg15]

The canonical response code for "you did something wrong but I can't/don't want to say what exactly" is 400.

I think if you don't want to supply even that, a better way would be to just close the connection and don't send anything back at all.

The only practical reason for a 999 error code I see is if you want to confuse the client about whether or not the response indicates an error at all. Maybe they were hoping some crawlers treat everything that's not 4xx or 5xx as "success" and so they can poison their index?

That thinking would be relatively naive though, as I think most http clients treat everything that's not 2xx as an error.

So most likely reason is probably some programmer that went through the REST fanboy phase and thought they were special.

[tubthumper8]

> I think if you don't want to supply even that, a better way would be to just close the connection and don't send anything back at all.

I think this is the correct answer if a bad-mannered crawler has been identified. To take it a step further, one could do a HTTP version of a SSH Tarpit[1] although that's likely taking things too far.

[1] https://nullprogram.com/blog/2019/03/22/