This just happened here. The last post on that page was attempted to post - it sent me back to a previous page in the thread and the post was not made. Back button in the browser took me to the new post screen, with the latest version of the post still in the text box, so I was able to re-submit without issue.
It's based on the time of your last page load anywhere on the site. Not the amount of time you were on the preview page. So if you have two tabs open, one with a post all ready and typed out, and another tab open browsing the site, your post will not go through if you load a page on the second tab and then hit submit in the first tab in under two seconds.
I am looking at the logs from your IP for that time period and you had a page load(looks like a quote button hit or something similar) one second before you submitted. From the looks of it you had another tab open and were scanning through the topic and hitting the quote button a bunch of times.
That's not to say this isn't a poorly designed feature by phpBB, but the cause is definitely what I said it was last page.
If it's clocking based on any tab you have open to the site anywhere, yeah that might account for it but it's shockingly poor design. Still pretty sure this has happened when I have one tab open, but w/e.
