I believe that there are people trying to get a court to decide that very thing. "Under God" has been in the pledge my whole life as well and doesn't bother me one iota.
Back to the original question though, assuming it's a bona fide mistake, how long before it becomes a tradition?
If President Bush decided on a whim to issue an executive order to afix a strip of white cloth with the word "GOD" to the top of every US flag, do you think it would fly? (pun intended) Do you think such an action would be construed as an unconstitutional conjoining of church and state? An argument could be made that this change would echo the sentiments of the pledge.