Overflow is a temporary condition that is remedied as soon as you make a call to TT_Update(). The only thing that overflows the queue is incoming camera frames that go unprocessed. Once the queue overflows you get frame drop and callbacks cease until you make another call to TT_Update(). By making regular calls to TT_Update() you avoid this and the resulting frame drop.
I've been looking into this issue closer and am testing and working on improvements for API callbacks in the upcoming release.