I think there is a bug in phpBB (currently I use 3.1.5) that causes a new post not to be posted or updated if you do not wait at least two seconds before submitting the form.
You did some research and find this code on in the posting.php:
This code makes no sense I think, because it assumes every post will only be submitted after a minimum of two seconds, which is not true.
If you are a very fast writer and you just update one letter, this will cause problems. The same thing if someone is fast and just copy and pastes his post from the local editor. I personally experienced this bug many times and it is very annoying, so I removed it.
Second is that there is an option which let you disable the waiting time before doing a new post. This option and a waiting time less than two seconds doesn't figure at this time.