Continuity mistake: During the "Worst pies in London" song, Mrs. Lovett drips the pie filling into the crust, and it fills the crust up to a bit over the top. In another shot, she rolls on the pie top, but the filling is much less than before, not even to the top of the crust.
Continuity mistake: Anthony's backpack during the I feel you secene keeps changing positions: Under the bench, slightly out, totally away, etc.
Continuity mistake: At the beginning of the song 'a little priest' Mrs. Lovette is holding the curtain and opens it. When the camera switches, the curtain is fully closed and she's just holding a little bit of the curtain.
Answer: His knife/razor/sharp edge to kill someone with.