Bad Emacs defaults
Author’s friend’s comment is funny:    ;; Basically, I was laboring under the impression that
    ;; `sentence-end-double-space' was telling Emacs the /right/ way to do
    ;; end-of-sentence spacing, but really it's just letting it know how sentences
    ;; work.  So setting it to nil lets you do sentence-level stuff, even on people
    ;; who are /wrong/.  So great.

I assume they were mostly kidding and this was only meant to be funny, but I learned the 2 spaces thing in college and then didn’t discover until a couple years ago that it had fallen out of fashion decades earlier, even before I went to college. IIRC the reason for the 2 spaces rule is because you need it in print, for non-fixed-width fonts, where the space character is really narrow. I think I even read that there never was a 2-space rule for fixed-width fonts? (*Edit just to say I think I (mis)remembered this story completely backward, and the corrections below are accurate. Print had different sized spaces already, typewriters need 2 to match.)Anyway. I loved emacs back in the day, and for a long time, but over many many years my .emacs file bloated up to like a megabyte. (That might* be a slight exaggeration, but I’m not sure.) I can’t maintain it anymore. I probably have a very very enormous list of bad emacs defaults.