It's extremely annoying. It doesn't give you any indication how you triggered it, nor does it give you any indication how long you need to wait until you can post normally again.

What's more, since the people who wrote the code for this feature are pretty smart, it's hard to believe these two things are oversights.

Edit: In response to a now deleted comment on how this feature is intended to be annoying, because your contribution was deemed harmful, but not ban-worthy, I say this:

I disagree that this approach is correct in that case. If you want to discourage certain behavior, then, on a site such as HN, you should treat your users as adults and tell them what you don't want them to do. Simply locking them out for an unknown amount of time, for unknown reasons, is just going to drive them away. This is just basic operant conditioning. Presumably, driving the user away entirely is not the result one wants a significant portion of the time.