Accidentally hitting the space bar
My biggest issue is accidently hitting the space bar when meaning to hit a character on the bottom row. This then inserts an auto correct word that I wasn't intending.
I appreciate that fat fingers and small phones will be at the root of this issue, but I suspect you can track how often this happens and decide if it is a wide spread issue. I wonder if there could be a clever option for peopleike me where the space bar had to be held a little longer, and if it isn't held long enough then the character is selected.
Something that needs to be borne in mind is that unlike touch typing on a computer, it is less easy to touch type on a phone and you don't see the output for concentrating on the keyboard...