"Only 1 comment allowed per 15 seconds; timer reset." is not working. It does not allow me to comment after 15 secs.
Is it bug or feature?
As you can see it says "Timer reset". It means that when you see that message you have to wait again 15 seconds.
The reasons for that implementation are unknown, one of those "Jeff said so" things. It used to be 30 seconds and used to be very annoying.
This is a dupe of: "Only 1 comment allowed per 25 seconds; timer reset."
Which is a dupe of: Really Long Comment Wait Time?
Which was deleted by Jeff Atwood.
Here is what it said, though:
And this was Jeff's answer:
And it doesn't end there, of course. That question was a dupe of: If I can't comment for 30 more seconds, why is the Add Comment button enabled?
The timer is reset every time you see that message. So if you jump the gun by one second, you must wait another fifteen.
This is another one of many protections against spam-bots and malicious users. Basically, it prevents a bot from repeatedly trying to comment because it will continually have the timer reset. If the timer did not reset, then the spam-bot would just repeatedly post until it worked (once every 15 seconds).
Although I've run afoul it in the past, it's not too terrible of an imposition. Just use the extra 5 seconds to stretch your arms or look out the window.