Most annoying auto"correction" your phone does
Mine is that if I type "person" it is changed to "per so." with the period and everything (except all these quotation marks, of course). When the hell did I ever type that? (I typed "per se" once and may have ended a sentence with it, but I wouldn't have hit an o instead of an e)
One thing I don't get (which rarely happens) is when I am entering a misspelling, I see the correct word showing up below and the autocorrect is going to use it, but then rather than replace the word with the recommendation, when I hit the space, it decides that it's keeping the obvious misspelling and adds it to the autocorrect dictionary for no reason. It already knew it was wrong!
Then the worst part... I once did that thing where you see an upcoming "incorrect autocorrection" down in the recommendations, so you put your finger on it, wait a moment, and it gives you the option to remove it from the phone dictionary.
I thought that was great, but it only worked the first time! It has never worked since, and I've tried it many times.