So there was a classic bug in the game NetHack where there was indeed a comment indicating there should be a fall through. Then somebody added a new case, and the fall through now went to the wrong place. But since the comment said it was supposed to fall through, nobody reported the bug.
There are linters that raise an error when there is a fallthrough without an explicit comment [0]. I feel like that should be a must for working with C code.
[0] I don't remember which one it actually was. Possibly something commercial for MISRA compliance.
https://nethackwiki.com/wiki/Yeenoghu#.22A_ludicrous_bug.22