Did we accidentally figure out the max post count for a thread?
No, but fun fact: when I designed / built this version of the forum software I never imagined a topic would ever have so many posts and didn't really know what the limit was myself, so I was kind-of a little worried that it was going to get to a point where it would implode. But it turns out that (when I checked) that it was only up-to about 30% of what it could go up to (before I'd have to build in a way for it to be extended in the background).
The issue seems to have resolved itself (it wasn't letting me get to the last page of the topic) so it may just have been something being cached somewhere, I don't know (difficult to diagnose when it's started acting normally). I'll double check and work on merging the posts in this topic back into the main one and re-activate that one when I get chance.
And for vNext, I'll make it truly unlimited :)