How can I remove video sticky stuff from a mirror?

We got a mirror a few months ago that the company put mask tape on within a big "X." They ripped it off briskly, leaving at the back bits and pieces (some big, some small) of masking cassette. It has since hardened so much that Goo Gone and WD-40 won't even touch it. It's not resembling I can even start to peel any of it past its sell-by date, it's that hard.

What else can I use to bear it off? Do I have need of to scrape it? Will that prejudice the mirror?
Keeping house heat?
Answers:    Just use a razor blade.
Shower Repair?
goo be gone is the surefire product for you