Failure of a C cell in a 4x battery pack, potential causes?
A reverse-inserted cell in a series chain of cells effectively is being charged by the other cells.
And it says very clearly on the battery &quot;Do not charge - may leak or explode&quot;  ;)
Take a look inside a young-age-grade toy's battery box and you'll find there's extra plastic or battery contacts added specifically to make it impossible to reverse-insert a cell, thus preventing parents from burning their kids this way.