I believe that's by design if you send an action request very quickly after a previous one. It's very easy to replicate. Open a post. Then click the upvote button and very quickly click the favorite button too. That will trigger it. I think it's used to rate limit.
>"Sorry, we're not able to serve your requests this quickly" is our little server process saying "help, I only have a single core and I'm out of breath here". If your account were rate limited it would say something like "You're posting too fast, please slow down."
dang, linked in one of the ancestor comments. But I still suspect you are correct.
