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();
    }

Maybe it was initially supposed to be a sort of "3-value boolean" (true/false/undefined) and not a standard bool. You can (rarely) meet this pattern in c++ if you use boost::tribool or in c# if you have a nullable bool. There is probably similar thing in other languages.

Sometimes this kind of thing happens after a few revisions of code, where in earlier versions the structure of the code made more sense: maybe several conditions which were tested and then, due to changing requirements, they coalesced into something which now reads as nonsense.

When making a code change which touches a lot of places, it's not always obvious to "zoom out" and read the surrounding context to see if the structure of the code can be updated. The developer may be chewing through a grep list of a few dozen locations that need to be changed.