I get something like that when I try to comment and then upvote too quickly.

I can confirm -- I've only seen this as a form of throttling (i.e. preventing users from sending too frequent HTTP requests).

There's a manual rate limit that can be assigned to your account if you post frequently on controversial topics. Afaik once it's there it stays until removed by a moderator.

I think that's a little different from what I described, but may be what the grandparent comment described.

I've been using hacker news for years and I think I've seen it <10 times too. Maybe our usage patterns differ and I only use HN off peak.