Sorry for the late reply, I did not receive any notification of your answer.
We are using the twitter ruby gem, but the exact error we received (Twitter::Error::Forbidden: User is over daily status update limit) is just a thin wrapper over the API answer from what I saw in the source.
I knew about replies and retweets, I tried to include everything in my count.
Luckily, we received the last errors just after I wrote this message and not one since then. I’ll reply again if I have more info.