That happened to me earlier when I had two browser tabs open for games and stepped away before I could post. After I came back, I posted the first then 15 seconds later the other. I think it's just a way to cut down on the spammers and also slow the rate of posts. No worries.
Kissing your lips while straddling your lap.
There was a time when you had to wait, best recollection 15 or perhaps 20 seconds, between forum posts but I thought that restriction had been removed. I will contact Gav.
I changed it last night as a temporary measure, as a spammer was bot posting in the forums every 3-4 seconds. Hundreds of posts, which I gave up cleaning up after their account was already deleted.
If we'd seen it earlier, we could have blocked the IP and deleted the member. But that is not a permanent solution to the issue.
There's very little reason for people to post more than once a minute, so I thought it would be a good way to slow them down while Gav works on a fix.
The trouble with all this kind of tracking, is that you are constantly adding resource upon resource upon resource, which bogs down the servers and ultimately will end in a poor user experience.
When they have moved on and realise this site is going to give them nothing, I'll put it back to how it was.
I can now see why many sites these days don't even bother with forums. They are one of the most time consuming, if not the most time consuming part of the site from an admin point of view.
And here I thought it was meant for me.
I'm joking but hopefully bots move on.
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Edit:
I tend to play in the silly threads where people choose to get easily offended and seeing as my answer are short, I have other tabs open to just bomb the threads.
So it makes sense for Nic and Gav to set a timer for the bots that have been spamming.
thank you Nicola and Gav!! 
..just kidding ???
am I the only one with one tab open ..it does help the seconds tick away
take care out there and Stay safe!