Editing a post immediately after posting it triggers the "Flood Control" feature of phpbb, and I'm locked out of completing the post edit until I wait a short time.
Yes, this happens to me all the time too. Especially I often figure I'd change something right when posting.
I lowered the settings, if the annoyance didn't go away then let me know. That was getting to me, too.
Is the "preview" button broken? 
I had raised it in an effort to control spambot floods, but it appears to no longer be necessary after what appears to have been a change to the registration page. I know forum.gbadev.org has implemented a mod such that submitting an edit to an existing post ignores flood control.