Generally when that happens it means that you hit submit at exactly the same time that someone else was submitting theirs and the script assigned both the same thread number. But whichever was saved to disk first is processed first, and since the other lacks the link to a parent post it is ignored when the message tree is constructed.

It would take me quite a while to locate it, so unless it was a really long post that took you a long time to write, it would be easier for you to repost it.