I do not have enough reputation to make my own edits. When I try to make an edit, e.g., to a title to clarify it, I get an error that I haven't made a change of at least six characters.
It seems that this calculation is only happening to the body of the message and not to the title box, so this seems to be a bug.
The work-around I've used is to arbitrary add enough spaces at the end of a paragraph in the actual post. Interestingly, this is enough to fool the character-limit calculation.
Update: @waffles tests provided enough of a controlled experiment to pinpoint the problem. In the examples where I was getting an error, I was making large changes to the title and also making, e.g., a one-letter typo correction to the body. The small edit to the body overrode the fact I had made a change to the title. If titles are to be edited more freely, my opinion is that this should hold regardless of any edits to the body.