One mechanism of double posting seems (to me) to be when there is an extraordinarily long 'wait' for the post to appear in the thread.
Another mechanism appears to be tied into the "egads!" notification (although that occurs in the former as well). When that appears, I can be almost sure that my response is in the thread. I just have to renavigate to WC to see it.
I think the lost number phenomenon is tied into the buffer somehow. I remember how if one posted #25 or #26, the response would 'disappear' only to reappear when the comment following it posted. Bara fixed that one, though.