The "pull off" parts are simply pressed into the metal (those parts are simply very thin)
your can might have had a leak.
(or it was pressed to deep, or it simply had somesort of a shock and cracked)
Result is oxygen entering the food, leaving you with fungus or bacteria growth, pressing the can open (the cheeseburger actually grows..
how interesting is that)
You actually see this happening a lot, it's actually a common ration problem.
That's also the reason why I don't buy issued rations (dented cans) with pull lids.
If they are not dented there is nothing wrong (99.9%) with the cans.
Yours might be dropped during it's life in the factory or supermarket, or accidentally during shopping.
With a bit of bad luck a small "tic" can already cause these kind of problems.
The cans are vacuum (it's filled when it's hot) and can be checked by tapping on it. When it isn't vacuum anymore you will hear a change in sound compared with the vacuum ones (can't describe the sound, but it sounds a bit "deeper").
Und warum rede ich eigentlich Englisch mit ein Deutscher?
(for the English speaking people; why am I speaking english with a German?)
Sure, someone may one day kill me with my own gun.
But they'll have to beat me to death with it because it's empty.
A computer once beat me at chess, but it was no match for me at kick boxing.
