If it's not flammable, why do some products have a spontaneous combustion warning label?

If it's not flammable, why do some products have a spontaneous combustion warning label?

Our Furniture Finish line has this label as a precaution. During the polymerization process, the oils will warm up naturally as they harden/dry. In extremely rare circumstances this process can cause oil soaked rags to catch fire on their own. To date, we haven't had a reported incident of this happening with our products!