It could be the boss, not liking the choice of the new COO, was trying to cause problems there or got the client "wrong" (meant someone else) or any number of things. Since all is quiet now and you are personally comfortable with the client feeling satisfied, I would let it drop as a mystery but be alert next time the boss raises a stink like that. It has been 6 months and the COO is no longer new/probably is not an issue for the boss (he realizes the COO is performing well and there's nothing he can do about it, etc.). Just don't trust your boss to that extent anymore since he has had this problem in your past.