The most egregious one I saw, I was tracking down a bug and found code like this:

    bool x;

    ...

    if (x == true) {
        DoThing1();
    } else if (x == false) {
        DoThing2();
    }

And of course neither branch was hit, because this is C, and the uninitialized x was neither 0 nor 1, but some other random value.