Two paths to the same error. Different mispaired bases (red) produce the same double stranded product once paired with complementary bases (green) so that each path leads to an ATG→AGG substitution error on one strand and a CAT→CCT on the other. The probability of these two errors is therefore expected to be very similar.
Rosen et al. BMC Bioinformatics 2012 13:283 doi:10.1186/1471-2105-13-283