For me [on Android, which is just as excerable] it's changing "has" to "had", which it does every fucking time. No matter how many times I change it back. I'll re-read what I've written and find I've been jumping between past and present tense like a grammatical kangaroo.

One thing that could vastly improve the suggestions would be to only insert a proper noun if I've first tapped the shift key. Here again, Android has the infuriating habit of suddenly inserting a person, city or country name into the most prosaic of sentences.

My other major annoyance with this kind of typing on mobile is that; once the fuckwitted algorithm has made a ridiculously wrong choice, when you spot it a word or two later and go back to correct it, all the corrections offered are variations on the wrong word it chose, rather than alternatives for what could have been chosen initially.

And the very nature of typing on mobile means you're looking at the keyboard, not what's appearing on-screen [as people can do, to various degrees, on 'real' keyboards]. So, unless you're looking up at the screen between every single word, to make sure it's correct, a lot of the correcting has to be done belatedly. So the errors tend to pile up.