The one the drives me mad the most is when I type a word which it incorrectly autocorrects to a proper noun (which gets capitalised), I then use backspace to delete that word and type the - completely different - correct one, but it insists on upper-casing the first letter to match the original casing of the incorrect autocorrect. It really should keep track of the fact that it introduces the case-change and invalidate it when the automated action itself is invalidated. This happens frequently to me.