Weak security in our daily lives (in English): basically, you can use a de Bruijn sequence to break into a car with keyless entry in what might be a non-ridiculous amount of time. I'm referring to the sort which have five buttons marked 1/2, 3/4, 5/6, 7/8, and 9/0, and a five-digit PIN that has to be entered. This trick takes advantage of the fact that the circuitry only remembers the last five buttons pressed, so if you press, say, 157393, then the car will open if the correct code is either 15739 or 57393. It is in fact possible to arrange things so that each key you press, starting with the fifth, completes a five-digit sequence that hasn't been seen before.
Of course, you shouldn't do this.
Via microsiervos (in Spanish).