During one of the flashbacks to 1745, a young Scotsman is eating what appears to be a Granny Smith apple. This is impossible as GS-apples were first grown in Australia around 1865, and only came to England around 1935.
(at around 9 mins) As Duncan is entering the hidden basement to talk with Methos, the light shining on the stairs shows the shadow of a boom mic operator that is lit from behind. You can see him move as he follows Duncan from the stairs and across the floor.