There is a detailed, and important, and terrifying article on Ars Technica, explaining how easy it is to hack the electronic voting machines increasingly being used in the US. Example (with my emphasis):
In order to use a supervisor card to access the AccuVote, you must first enter a four-digit PIN. In version of the machine that was in use as late as 2003, the exact same supervisor PIN was hard-coded into every single AccuVote TS shipped nationwide. That PIN was 1111. (I am not making this up.) This is still the default PIN for these machines, although the county can change it on a machine-by-machine basis if they have the workers and the time.