Every database needs to have data added to it in some way, and at that point of entry, mistakes can creep in.
Once those mistakes are in the system they can be hard to spot, resulting in error laden results.
To date there is no 100% guaranteed foolproof method for entering data into a system which is completely error free. However there are techniques that you can use to help you reduce the errors to a minimum.
One of these techniques is called Validation.