As a general rule, taking money that does not belong to you is illegal.

Who rightfully owns the money is determined not by the programming of the vending machine but by the conventional social arrangements of society.

i mean, it was purposefully acquiring money that didn't exactly belong to me. the Right Thing To Do would have been to report the error and get it corrected, instead of leaving it open for exploit, like when my friends or I needed quarters for laundry.