The red and white panels of the Texas flag are incorrectly inverted in the liquor store scene with Brian. The white should be on the top of the flag and the red on the bottom.
When Brian fires the revolver the sound of the empty bullet shells hitting the ground is heard. Empty bullet shells are only ejected with semi-automatic handguns, not revolvers. Also, it is a six-shot revolver and he fires seven shots.
When Lois takes the cake out of the oven she starts frosting it immediately. Cakes need to cool. Also, when she carries it to the table, she's wearing one oven mitt and holding what would still be a hot pan with her bare hand.
Peter mounts the horse from the wrong side.