I ran across the 30,000 character limit in this answer, and while working to pare my 42,000+ characters down, I found the "on-the-fly" character limit pop-up quite helpful.
However, that pop-up did not do the same count as the submission count.
After having dropped down to about 30,008 characters and making a few last changes, the pop-up went away and I thought I was good to go. I hit "submit" and then it told me I was at (roughly, I don't remember the exact number) 30,250 characters! 250+ characters more than the "on the fly" count.
So the "on-the-fly" count and the "submission" count are not the same.
EDIT with Nathaniel's analysis
Here's the on-the-fly count shown after adding about 300 characters (exceed by one character):
But here's what shows if one character is removed and an attempt to submit (exceed by 289 characters):