Mechanical combination locks can be difficult to use for people who have never used one before. The most common mistakes are forgetting to pass the first number when dialing the second number, and rotating the dial backwards. In an electronic combination lock, the batteries can die, preventing the lock from opening until the batteries are replaced. With most combination locks, it is possible for a thief to obtain the combination by watching a legitimate user entering it. On an electronic combination lock, a thief can sometimes guess the combination based on the amount of wear on the buttons. An example of worn buttons can be seen in the photo on the right. More-modern keypad locks require the user to press one or two other buttons indicated by LEDs after the combination is entered. This causes the other buttons that are not part of the combination to wear down as well, preventing a thief from determining which numbers are part of the combination based on wear marks.