On the iPhone 5 I have come across an editor bug. The way to replicate it is to start typing and then reposition your cursor on the screen. Then go to type and you can't. Nothing happens when you try and type.
The only way currently to fix this is to press done and then click in the editor again.
Yeah I have noticed this as well. I noticed it mainly when trying to tag someone as it essentially does the same thing you are moving the "cursor". I am not using an iPhone 5 but I am one model lower on the 4s.