The second happens rarely in my experience, but it does happen. The first is what I was wondering, because both the sound and feel would be off even if you aren't looking.

In my experience, manufacture or storage problems, usually the tab is structurally compromised so it breaks under stress instead of breaking the seal.

Gotta remember shotgun, whenever that happened I'd try to go "old school" (remembering those old cans that required a lever opener) and open it with pliers or something, only to get a faceful of fizz or beer foam.

They got those beverage cans down to a science.