We present a concrete situation where there is a difference between the theoretical security of a cipher and its limitations when implemented in practice. Specifically, when entering a PIN, smudges on the keypad substantially reduce its security. We show how the number of possible keys in the presence of the "smudge attack" can be calculated. Specifically, it turns out that in all interesting situations, a key with fewer smudges increases security. Possible extensions for use in an undergraduate discrete mathematics or cryptography class are presented.