It's in the last series, one of the last episodes. Mike and Phoebe are in the Coffee House (naturally) and are sitting at one of the high tables rather than were they all normally sit.

They're having a conversation and the camera is switching back and forth bewteen them. When Mike is talking the first time, his scarf is hanging loosely around his neck, went it switches back to him, his scarf his tied in a knot. then hanging loose again next time, then tied again.

Blatantly obvious - someone should've got fired for that one!