Chance of Player Scratching Discs?

[Yuzo]

whats the chances of a PS3 console scratching or damaging a disc while playing?

... apart from obviosuly knocking over the console when in use.

im asking because a friend had a dvd player that used to create slight hairline scratches on discs.

which made me wonder;

.. how could a player create scratches or damage a disc, when it just shines a laser onto the spinning disc?