* Poor image resolution, usually because the plate is too far away but sometimes resulting from the use of a low-quality camera.
* Blurry images, particularly motion blur.
* Poor lighting and low contrast due to overexposure, reflection or shadows.
* An object obscuring (part of) the plate, quite often a tow bar, or dirt on the plate.
* A different font, popular for vanity plates (some countries do not allow such plates, eliminating the problem).
* Circumvention techniques.
* Lack of coordination between countries or states. Two cars from different countries or states can have the same number but different design of the plate.