Here's an earlier thread I had about issues with frozen film. I eventually had the negatives sent to Ilford for examination. The exact cause remains "inconclusive", however the best explanation was that freezing/defrosting - ie moisture - was the only likely potential cause. Since this issue first raised its head, I have witnessed enough of the problem that I would say that 35mm film shouldn't be frozen in its plastic canister ONLY - Im trashing all my film stored this way as its too unpredicable. Other rolls purchased at the same time and having gone through the same identical treatment, but remained in their cellophane and cardboard packaging of bricks of 10, appear to be fine.