The answer is proofreading feature of DNA polymerase. As the DNA polymerase III replicates DNA, it
has a proofreading mechanism in one of the
subunits of the holoenzyme. This subunit detects a bad conformation in the double
helix when the wrong base pair is placed.
In this case, the DNA polymerase III backs up
a bit and waits for the exonuclease to nick out the misplaced base.