I once found an old tin of black treakle at the back of a little used cupboard. It felt like it was half full so I started to lever off the lid with a spoon handle. The next thing that I knew was a loud bang as the lid hit the kitchen ceiling!
It looks like it had been fermenting for the last 5 years or so and the lid had been stuck on with old hardened residues. I should have noticed that the bottom was bowing out!