Many writers use autocorrect in order to help with spelling or even grammar, but most times the grammar check will only denote passive voice and sometimes it can completely ruin a piece of writing. As a sci-fi writer, sometimes the names of my characters give the spell-checker fits.
All of us who use iPhones know how auto-correct can give people the wrong idea.
Taylor Mali, one of my favorite poets, wrote a poem about the problems of autocorrect. Unfortunately, as my students often find out, autocorrect can sometimes produce some rather rude and unintentionally risque results.
The main reason fiction writers should turn off autocorrect is due to style reasons. The biggest problem is that some writers (not myself) rely too heavily on the spell checker rather than doing honest and careful revision. Hopefully, if you are doing the NaNoWriMo, you are making time to think about what you write and planning to finish the novel by at least the 25th so that you can spend some quality time revising.
It will be worth the effort...and the time.