The reason for this error is that most likely you have release the disc from ps3 too quickly. This normally happens when, say, you have just turn on your ps3 that has a disc left in the drive, and by instinct you tried to remove it quickly by pressing on the eject button. You see, whenever you left a disc on the optical drive while turning on the ps3, it will somewhat try to read through that disc, and since you removed it quickly while it is still reading, you caused it to get "upset". To the ps3, it would be like it got left reading somewhere halfway, and will inform you that it encountered a startup reading error.