The slip-up occurs during the movie when Pirelli is speaking with Todd, in Todd's shop. After the contest, Toby and Mrs. Lovett are speaking about life and Mrs. Lovett gives Toby a meat pie. Toby takes a few bites of the meat pie in between speaking with Mrs. Lovett, you'll notice the bites out of the pie, the scene will cut to Lovett and then back to Toby, when the scene returns to Toby his meat pie has miraculously regenerated and is 100% whole again.