This is a note for users of OS X Yosemite. With the new OS, spelling autocorrect is on by default. For me, at least, this was most evident in the mail app (almost all my other writing occurs in Emacs). When writing function names such as getkey-function, autocorrect would helpfully correct that to get key-function. It happens so quickly that half the time I didn’t even notice it.
I finally got fed up enough to try to fix it. The obvious place to look was in mail preferences but all you could do there was to turn off spell checking completely or just do it when you press send. I asked Duck Duck Go about it and found this helpful page. The TL;DR is that you can turn off autocorrect in the keyboard preferences; just uncheck the autocorrect box and you’re done.