I’ve seen this a few times, where the content of the first comment on the bbs will leak into “Show Full Post” text. Latest example on: Biopic of “little old lady” folk artist a hit in theaters
You can see lucy_gothro’s wikipedia content is appended to the article text. It’s not just oneboxes, though, I’ve seen just a straight text comment either append to, or completely replace the article text.
I’m on Firefox 53.0, windows 8 currently, but I’ve seen this on older versions of FF, and windows 7 at least.