Well, I had a recent experience with a tin that would not open. Got some advice, everything from using a vice, a hammer and thick screwdriver, but someone gave me an answer which was logical, simple and lacking in danger.
Yes, this baby was beyond simply tapping around the lid to break the seal.
If you have a tin that will not open, take a hammer and small nail and poke a single hole in the lid. This releases the vacuum pressure and the lip pops off like a charm.