For the last twenty years or so, I’ve been rendering the em-dash (and the occasional en-dash) with — mostly because I could never remember how to spell the damned entities. (And typical usage around here has spaces around the dash, which is neither technically nor typographically correct.)
WordPress, as of version 4.3, has decided that I will no longer get away with —. It’s converting that string on the fly to —, which is a correct em-dash in Unicode, and it displays the way I want it to display. Only thing is: now I wonder what else WP is doing behind the scenes.