The limit is set to 50 posts a day. It's actually there to prevent spam instead of encouraging sign-ups (although it works in the same way).
We have occationally in the past had people using the forums as a messaging system, although it's been a long time since anyone's reached the limit.
Thanks for pointing this out, it should probably just show a 'limit reached' message or something instead of redirecting to the upgrades page.
5 years
