I was introduced to this idea on the comp.risks forum, where it was presented as an attack on an answering machine with a 5-digit passcode, which, naively, would require an average of 50,000 guesses of 5 digits each, or 250,000 key presses. But this particular device not only didn't lock out after some number of incorrect guesses, it didn't require a separator character between guesses. It just listened for the correct five digits consecutively, regardless of what you had entered before. So a superpermutation just 153 digits long was sufficient to crack every device.