An example where a single HOX gene mutation leads to a set f extra wings in Drosophila.
http://drnelson.utmem.edu/MHEL.hox.html
Mutations in the 8 genes of the HOM complex cause large scale mutations in flies. A mutation in bithorax causes a fly to have an extra set of wings. Mutation in antennapedia causes a leg to grow where an antenna should be. These genes are not master switches for making wings or legs, but they specify position in the fly's body. The order of the genes on the chromosome is the same as the order of segments in the fly's body where they are expressed. The left most gene is expressed in the head, the right most gene is expressed in the abdomen. When a gene is deleted or mutated, the segment where it is normally expressed cannot tell where it is because its position clue is gone, so it behaves like the closest segment to it. That is why a bithorax mutation causes an extra set of wings. The segments adjacent to the bithorax segment dictated what should be made.
I know there are other examples of mutations in a single gene or gene regulatory region that result in extra limbs but I don't have time atm to look.