Continuity mistake: When 9 meets 3 and 4, they open his zipper. During this scene, several close-ups show 9 with the zipper closed and open again without anyone closing or opening it.
Continuity mistake: When 9 created the "torch" from a light bulb, it needed to be turned to work. However, when 3 and 4 investigate the bulb one of them simply puts it in and it starts glowing without being screwed.
Continuity mistake: During the funeral scene for 2, a tunnel with steel bars is seen in the background, with none of the bars broken. When the camera angle reverses once 2 has floated into the tunnel, one of the steel bars is broken.
Continuity mistake: When the Seamstress is sewing 1, he falls and the coin falls off his hat. However, in the full shot, 1's coin is back on his hat.
Other mistake: When 1 strikes off 2 (I think) from the calendar we can see that 1 is Wednesday, 2 is Thursday, 3 is Friday and that 4 is Saturday but 5 is Monday; meaning that Sunday has no date.
Continuity mistake: Little metal washers are seen on the table behind 9 when he comes to life. There is a wide shot later, and they are gone.
Continuity mistake: After 9's unleashed the Talisman on the killer robot, 7 runs over to congratulate him. 7 puts her hand on 9's shoulder, but then in the close-up of 9, it's gone.
Continuity mistake: 9's zipper does itself up after 2 has inserted the voice box in him.
Answer: Simply a character choice to not sew 1's mouth.