When Amanda and Duncan are talking about Zachary at the circus, Duncan takes a rope from Amanda's hands and throws it away. Then, when he says "You should leave town!", it's suddenly back in his hands.
During final sword fight with MacLeod and Blaine, the power cable used to make the swords spark when they strike each other can briefly be seen under MacLeod's feet.
When Methos is in the shop looking for a sword he asks for one that is Damascus steel. The one that he is given that is supposed to be Damascus is not. The way Damascus is made it had a distinct one of a kind pattern and the sword he is given does not.
As Amanda and Zachary are robbing the museum, Duncan escapes from the circus and secretly arrives to witness Amanda stealing the book. Neither Amanda nor Zachary show any signs of being aware of Duncan's presence until Duncan turns the lights on, showing surprise he's there. Since Amanda and Zachary are both immortals, they should have sensed Duncan either approaching or entering the building.