When Cliff and Alex go hunting by a river, they each have a beer. Alex sets his down to take a few pot shots and it falls over and dumps its contents. The foam is clearly visible. Cut to a different (frontal) view of Alex. When the boys get up to leave, the beer is standing up and Alex picks it up as they walk away.
As Cliff gets out of bed to answer Lily's late-night phone call, the boom mic can be seen moving closer to the doorway.