There is one type of Mercenary (knife wielding user) which some of them appear as an invisible entity, you can't target them since they don't have a model but coretech allies can damage them without trouble, they still use hypos and perform their abilities (crippling strike) but it don't affect you at all.
Unfortunately I forgot to save the game and post screenshots just in case but this has happened to me at least in two playthroughs.
