Once a week I photograph adoptable animals at the local humane society. We always shoot against a white seamless background. To mix things up, typically we throw a gelled speedlite on the background. Sometimes I get a bad colored halo around the critter. I have the pup as far away from the background as I can, typically 4-5 feet, as we are working in tight spaces. The speedlite has a Honl gobo on it to prevent spill on the subject. The speedlite on the background is about 4 feet from the background and the power is turned way down (PW AC3). I don't always get the halo and am baffled as to why it occurs only sometimes. Any ideas would be greatly appreciated.
A couple of (small) examples can be seen on the humane society's website here and here.