The prologue is set in the Dark Ages, (the centuries of barbarism which followed the collapse of the Roman Empire), and Merrick later confirms that this was before the Crusades. However, the costumes depicted are from the medieval period, which came many hundreds of years after the Dark Ages ended.
After Pike talks to Buffy while on his motorcycle, he does a turn toward the photo booth. You see a close up of him just as he his revving up his bike, and he is maybe two to three feet from the photo booth, which is not enough room to do the maneuver. In the next shot, you then see him about six or seven feet from the booth, and he does the maneuver.
During the cemetery scene with Buffy and Merrick you hear a loon calling on more than one occasion. A loon would be on a lake, not in a cemetery. They can barely walk on land, and they only take part in calling during the morning and evening hours, never late at night.
When Merrick takes Buffy to the graveyard he shows her Robert Berman's grave and says he was buried 3 days ago. Yet the grave already bears a carved tombstone which would not have been possible in such a short time.
The goof items below may give away important plot points.
Lothos is seen playing a violin during Amilyn's death scene. However, it is impossible for any sound to have been coming from the violin as Lothos is wearing gloves which would have muted any sound coming from the instrument; the violin is also an electric violin plugged into absolutely nothing.
When Pike is holding Benny against the electrical breaker box and Benny is apparently being electrocuted, Pike appears unharmed. Since electricity seeks its ground by the easiest possible route, Pike should also have been electrocuted along with Benny.