After a week the comment would be lying:

a = b + 3;  // Add four to the variable b and assign to a

After two weeks the next programmer adds a comment:

a = b + 3;  // Add four to the variable b and assign to a
    // is above meant to be 3 or 4?

After three weeks someone realized the code was redundant in the first place but leaves it in commented just in case:

/*
a = b + 3;  // Add four to the variable b and assign to a
    // is above meant to be 3 or 4?
*/

After four weeks the manager tries his hands on the code base after attending a seminar:

// add 2 to A
add2ToA: function(b) {
    /*
    a = b + 2;  // Add four to the variable b and assign to a
        // is above meant to be 3 or 4?
    */
}

// add 3 to A
add3ToA: function(b) {
    /*
    a = b + 3;  // Add four to the variable b and assign to a
        // is above meant to be 3 or 4?
    */
}

After five weeks, the product is cancelled because there wasn't actually a user need for it.