If a player has a CPM skin applied to them and another mob takes damage, the first person player's hand turns red. This can easily be reproduced by placing a zombie/skeleton during the day when they take damage. If the player does not have the CPM skin, then nothing will happen to the hand.

Bug:

There is no bug if the skin is not applied:

Also, this can be reproduced even only with Customizable Player Models, no optifine, fastcraft, etc.