If you write a reply to a post while (unknown to you) another member is also replying to it, and if you submit yours after his, your submission will not be added to the thread. After you hit "Submit", the first member's submission will appear on your refreshed screen along with your now yet to be submitted post. It seems like it has just been added, but it hasn't; it actually requires a re-submission. If you fail to re-submit at that point and navigate away, your intended post is lost.
I know it sounds confusing, and it took me a while to realize what was happening. I thought some of my posts were just vanishing; instead, I discovered they were never actually added.
![Rolling Eyes :roll:](./images/smilies/icon_rolleyes.gif)