what do you see?

OK, here's what just happened. I got back from work/shopping and innocently decided to reheat some leftovers from last night, in a plastic tub type-thing in the microwave. But perhaps I let it go on for too long, while I was occupied with something else and consequently it got too hot and the lid somehow became sealed on and sucked in, extremely, extremely tight. I mean hermetically sealed round the edges, with not even a visible join. Cooling it under the cold tap for a while didn't improve matters, nor did turning it upside down and shaking it, so I ended up sitting on the floor with newspaper (see, I do think ahead!) repeatedly smacking it with a hammer to release the vacuum.
Now, is this normal? Is this the kind of thing that usually happens to people?