Never find fault with the absent.

Criticism aimed at someone who is not in the room costs you nothing and gives them no chance to answer, which is exactly what makes it cheap. It also teaches everyone listening what you are likely to say about them later. The habit feels like candour while it is happening and like a small betrayal afterward. You gain agreement in the moment and lose the sense that you are safe to be absent from.

Catch yourself at the start of that sentence rather than the end. If the point is worth making, save it for the person it concerns.