You know how irritating it is when you just posted a message and then you discover that it's full of typos or you've left out the most significant word? Just now I posted something and, by mistake, clicked the arrow on my computer that take me back to the previous screen. The message was unposted but remained in the box at the bottom. Consequently, if you have made a mistake, you can unpost and correct it before posting again. Perhaps other people have already discovered this – but I didn't know.