Continuity mistake: At the end of the "I feel you" scene, Anthony walks with his face covered in blood. The hair around his forehead covers at first, and suddenly it doesn't.
Continuity mistake: After Mrs. Lovett says "A customer!", the knife she stabs on the counter and the blob of flour change positions.
Continuity mistake: When Sweeney kills Turpin, his forehead is covered in blood. Half a second later there's just a tiny spot.
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: It's a common colloquialism in England, particularly with London and East Midland accents - people often pronounce 'th' as 'v'. It's poor english but very common, and certainly would have been common in the lower classes in London at that time.