The classic case of this is the indoor cat who is highly aroused watching another cat come into his yard, and up to the window that is keeping him in. The indoor cat wants to attack but cannot. Owner walks past and the indoor cat attacks the owner. This is redirected aggression. The aggression needs an outlet, and finds it in the form of an accessible target.Displaced aggression is more accidental. Here a 2nd indoor cat may walk up to the other cat at the window, which takes cat #1 by surprise and cat #1 strikes out at cat #2. This is a common scenario which can lead, if not addressed properly, to prolonged aggressive problems between cats in a household.