Web out does not leave any visible residue. However, it does contain surfactants which are basically soap. What can happen, is that when you spray, the surfactants can "clean" the areas on which they are applied. So if there is any dirt, debris, dust, etc, and the Web Out runs or drips, it can clean those areas an appear to leave a stain.
I recommend to wash or power wash any areas first, let dry and then apply it. The EcoSafe CobWeb Cleanup
does the same thing and is a little better and killing spiders on contact.