During the song "My Friends", Angela Lansbury accidentally says about Todd's razors, "them blades is chased silver" rather than "them handles" (as per the libretto). A razor blade out of silver would not only be less effective than the carbon steel commonly used, but would also be ludicrously expensive for such a mundane task.
During the song "My Friends", Sweeney's razors are mentioned as having blades made out of chased silver rather than having silver handles. A razor blade out of silver would not only be less effective than the carbon steel commonly used, but chased silver might result in a serrated edge over time, as well as just being ridiculously expensive and wasteful for such a mundane task.
As the company gathers in front of the oven in "The Ballad of Sweeney Todd", the camera catches Sweeney moving into position for his entrance.
The Beggar Woman's mouth does not quite move in time with her final verses in "Johanna" (Act II).
When Toby finds the dead Beadle, his words do not match his mouth at all.
At the end, during the curtain call, a camera man can be seen in the rafters, stage left.