At this stage, this Phab task is probably more "FYI" than "actionable bug", but:
I posted a new thread on 9 December 2015 at mw:Talk:Phabricator/Help. The straight single quotation mark in the topic title was percent-encoded upon saving. When I refreshed the page, it displayed correctly.
I've not been able to reproduce this since. It originally appeared in Safari 9 on Mac OS 10.10. I've also not seen any other reports of this happening.