It's a bug in the specification. It happens sometimes. But the tests only test the correspondence between the test and the code, not between the code and the intention.

Say:

fn is_even(x: u32) -> bool {
   is_odd(x + 1)
}

fn is_odd(x: u32) -> bool {
   x != 0 && is_even(x + 1)
}

the bug in the intention is not very easy to see. (And, by the way, is_odd(2) produces 100% code coverage, which says something about that metric).