Continuity mistake: When Mary falls down the hill, her glasses land in the grass flat against the ground. In later scenes, the glasses are standing up with the arms of the glasses stuck in the grass so that the sun can shine through them to light the grass on fire. (00:49:00 - 00:55:00)
Other mistake: After the stagecoach crashes and Mary discovers that neither Marge nor Adam can move, she tells Adam that she's going to leave the canteen for him and Marge. Neither one can move. Neither one is going to get the canteen to the other.
Continuity mistake: After the stagecoach falls and eventually lands, we first see the side of it (where Adam is) and it's fully in the shade. Then, in a later scene, the same side is fully in the sun, and then eventually in the shade again.
Visible crew/equipment: When Charles takes off in the wagon after talking to Garvey, you can see the shadow of a boom mic on the ground amongst the other shadows for several seconds.
He Loves Me, He Loves Me Not (2) - S6-E26
Other mistake: Charles has just brought Adam a letter to the site of the blind school rebuild. When Adam is told that his father died, he and Mary walk away from Charles. The terrain was very uneven. Being blind they realistically wouldn't be able to navigate it without stumbling.
