I've had this a few times now, usually when I paste in the description. The remaining character count says I have characters left to use, but when I Submit, it fails with the "Description is too long; maximum 5000 characters" error.
Please see the screenshot, and note that the actual character count in the pasted text is 5024 characters (checked in a text editor). So the Submit error is correct, but the edit box counter is somehow not counting right.