Your diagnosis is correct. I am not getting any errors using Firefox or Opera, but only IE 8.
In advanced Internet settings both "debugging" boxes are checked, and "display notification about script error" is not checked. Is there anything I can do to prevent the script still displaying the error?
It probably won't help much but I have uploaded a slightly bowdlerised version of the page where the error is appearing:
this has some element disabled, so no actual entries will be made in the database, but for me at least it's still displaying the error behaviour I have described.