Here some info some may find interesting. Although primarily about pheasants, has interesting things about mutations and color genetics. It was a 3 legged partridge and not a quail.
http://aviangenetics.com/PheasantGenetics/genetic_effects_of_domestic.html
Though not that uncommon, thank goodness it's not that common. ;)
The thread I was talking about, the quail had 3 legs. It may have been in a scientific journal article, review but I thought I saw it on here? I'm the worse for trying to remember where I have read something much less trying to...