Proofreading is traditionally thought to be the checking of copy to detect misspelled words, bad grammar, etc. This is important, but someone should also take the responsibility to do a conceptual proofing of the piece.
Conceptual proofing is stepping back to see the big picture so no "global" mistakes are made. You cannot do this by simply comparing the copy with what was submitted. You must go back to the original source. Some examples:
Be careful. Making a global mistake – such as a wrong phone number or web site for your housing firm – can be very costly.