it happens sometimes when copying and pasting text from one place, generally a software that isn't text based, to another. it also might just be their computer/keyboard but basically what happens is when the text is posted, symbols that are also used in coding (apostrophes, ampersands, brackets, etc) are interpreted as their code names instead of as themselves.
It's in other people's posts. But I remember seeing it for every single contraction in every single post a few times in Brewsters (these were old posts).
But your explanation explains the one I mentioned!
This was from back in 2011 when TBT moved from Zetaboards to vBulletin. It's just from issues converting between characters sets when the site moved. Hopefully, when TBT moves from vBulletin to XenForo shortly, the same issue doesn't happen. I doubt it will, though!