I was scratching my head as to why my bbPress forum was displaying topics with negative numbers. It seems to coincide every time someone deletes a topic. The number of topics can sometimes falls below zero into negative territory. It just looks odd to have a forum with negative topics.
In any case, the fix is to login as Admin and perform a Recount. Just go to Manage > Recount. I checked on the first 7 boxes. I skipped boxes 8 and 9 that either delete or remove posts with no topics, etc. It only takes a few seconds to perform the recount. Once recounted, the topics would show the correct values.
I never did get a clear explanation as to what is causing the negative numbers to show up. There seems to be a good discussion at the bbPress forum, but it seems the question was never resolved.