Recently one of teachers noticed that some Assignments that had been copied between courses would show the assignment introduction as a plaint ext box with html markup visible.
I tracked this down to Moodle recording the wrong format for the intro. To fix all instances of this in the database I ran the following query:
UPDATE `mdl_assignment` SET `introformat` = 1 WHERE `introformat` = 0 AND `intro` like '%<p%'
This updated the format for the 1400 or so assignments that had been copied and had this problem.