Wham, Bam, Thank You, Mammy - S6-E5
Continuity mistake: When Mammy Watkins first hands the letters over to Blanche, the ribbons holding them together are knotted but the knot is on the bottom of the pile, and the strands are quite long. When Blanche reaches for them, the knot is on the top, it is now tied in a bow, and the strands are much shorter and neater.