Check this out:

An 81-year-old woman with a mercury-zinc powered permanent pacemaker experienced the sudden pain on her pacemaker pocket followed by an explosion. We are aware of no other report of the spontaneous and symptomatic bursting of a generator battery with fracture of the pulse generator capsule.

Emphasis mine. It’s an old paper (1987) and you can get the full text for free at the journal’s site, here.

Crazy, huh?