The Editing Mode switcher. It typically looks like a pencil (when you are in reading mode) or a page/book (when you are in editing mode). It confuses me in its normal way of working.
It would be nice if there was an option to change it so that it reads the opposite way, ie: a pencil to me suggests editing mode and a book suggests reading mode. I know that some appearance themes change it like this, but it would be nice if it appeared in the setting such that a theme change was not required.