Re-Fixed my EOS mount Tokina 19-35mm lens the other night. I had picked this up for a song a long while ago, and one day while using it, the rear metal ring insert which goes around back element, but is within the metal mount itself, popped loose. In my haste I quickly took the rear mount off, and super glued it back. Worked fine in a non full frame camera, but would block the mirror on a standard film Canon.
I realized that the ring I glued back was reversed and protruding out of the mount. But I had used superglue in my inexperience back then and it was really stuck on there tight.
I did a quick search on removal and discovered superglue or cyanoacrylate becomes brittle in cold temps. So instead of throwing it into the fridge, I blasted the ring with liquid CO2 from a compressed air can by holding it upside down. Bingo, easily snapped off after a two applications. I flipped it and inserted it again, and all is well, the extra glue on the edges is providing a tight friction fit now.