'Spamming is not permitted!' warning

Just had a weird thing happen in the Duplicated entries thread, when I posted a short reply to a report of a duplication there. As soon as I hit 'Post Reply' a big red warning anti-spam message popped up saying that I had exceeded the number of posts allowed in the time span, and the message had not been posted! The post did go through however despite this warning, but in more than twelve years here, and often in the early years posting many more times in a similar time slot than I have tonight, this has never happened before. Any ideas what might have triggered it? I have only posted half a dozen times at the most in the last three hours, and they've been well spread out over that time too, which I hardly think is spamming. 

Edit: This was the URL of the warning message I received: https://www.progarchives.com/forum/not_posted.asp?mode=maxS.

I searched and found four earlier reports in this section of the forum of the same thing happening - a couple from 2005 when the site was in its infancy, one from 2011, and then another one from 2016. It therefore seems to be a pretty uncommon occurrence, and the strange thing is that the post always seems to go through ok anyway, despite the warning.