Twice now, in two separate games, both on the map Adaptive Millenium, I have seen percies that could not be damaged. They weren't under shield, etc.; in one case I set a GC on a percy and it took no damage. Just sat there at full health shooting back.
I am guessing it's a map issue. Is the best thing to do to report it to the map maker?
After some time (when I had reasonably determined the unit was invincible and it would have been very dead if it was not) I asked the owners to ctrl+k the units, and they flat out refused. Is that reportable? I know it's not malicious, but to not suicide the unit after you're aware you are inadvertently using an exploit seems like there should be some sort of punishment. In one case a percy was standing on a wreck and we could not reclaim the wreck because it would kill any engies that approached. I told the owner the percy would be dead if it wasn't invincible (as I had shot at it with a GC for a good 30 seconds) and asked them to suicide it and they refused.