There are two ways to edit a submitted post, a "full" edit in a separate page, or a "quick" edit in-place:

The problem is that the quick edit will mess up any Unicode characters.
For example, the following string uses special quotation marks:
“Example”
But when after I edited this post in-place, and saved it, it was silently changed to:
%u201CExample%u201D
This is a big problem since many people paste text from word processors, and the edit box shows no indication of the problem.
The "full" post modification does not exhibit this problem.