"If I were a gambling man, I'd bet it's due to two posters posting at the same time (not necessarily to the same thread), with one of the messages pre-empting the other. In other words, it's probably something entirely outside of your control."

I have had that happen. A post I try is 'hung up' and so I copy quickly and use the back button. Try again and boom, Bob's yer uncle. There is a difficulty when two people post at the same time, something that happens often.