El Capitan auto-corrects even though I turned it off in settings I notice that things are being corrected even when I turn spell-check off. This happened on both of my computers, and particularly stupid spelling corrections are taking place, only since upgrading to El Capitan.

Example.. I updated a ticket at work "This is now on production" .. when I submit the update, I see it says "This is not on production" ...

At first I thought maybe I did a typo but this keeps happening.

Every single instance was where the word is actually spelled correctly however it replaces it with another word.

I've turned spell-check off so I am confused as to why it's doing it at all.
There is only place to disable spell check in System Preferences and on doing that it works fine for me on El Capitan. Can you try enabling and disabling again and probably a reboot to see if it helps.