“You're posting too fast–please slow down.” Is there a one submission/day limit?

I had this happen to me after I posted something the admins thought was boneheaded and they were right. I promised not to do it again and I got restored.

"Please don't post on HN to ask or tell us something. Instead, please send it to hn@ycombinator.com." https://news.ycombinator.com/newsguidelines.html

That said there's users submitting 10+ pages per day so I don't believe there's a fixed limit.

It is likely an anti-bot rate limit. It is likely a limit of one submission per five minutes or something.