At the start of the film, as Todd's ship arrives, a completed Tower Bridge is seen. Construction of Tower Bridge started in 1886 and it wasn't completed until 1894, at least 50 years after the film is set.
Mrs Lovitt gives Toby a meat pie and he's nearly done eating it when she sits down and begins talking about her late husband , but in the next shot after Toby takes his wig off the pie has only a few bites taken out of it.
During "My Friends" the razor in Sweeney's hand goes from fully open when he first opens it to half way when he stands up, and back to fully open when he turns around.
Also, his grip on it changes from fingers on the blade in the close-up to fingers on the handle only when the camera cuts to a further view.
When Mrs. Lovett is singing "The Worst Pies In London" she is seen pressing on dough on her counter from behind her hand. She leans over to push on it, and the shot changes to in front of her, where she leans over to push on it again.
Johanna is seen packing stuff neatly into a suitcase , she puts the last item into her suitcase neatly , but when Judge Turpin opens her door we see the item hanging over the side as if it was hastily thrown in.
After Todd said they'd bury Pirelli in a secret place, Mrs. Lovett goes to the window and pulls back the curtain. In the next shot, we see her through the window with the curtain moved to about half its pole's length. In the shot after that, however, the curtain is fully closed and she's merely lifting a corner of it.
When Toby is in the basement at the end, he takes a very small bite from the pie, then pulls a piece of finger out of his mouth far bigger than the bite he just took. In the following shot, the bite in the pie is shown much bigger, almost big enough for the piece of finger he pulled out of his mouth.
At the beginning of the Market scene, Mrs Lovett states that it is Thursday. However, a few scenes later, just before Pirelli arrives at the barbers, she says that it is Tuesday, when, supposedly, no more than two days have passed.
During one of the climactic deaths scenes at the end, Sweeney slits the victims throat. Blood spills all over the windows on the roof, The next shot is of Sweeney but the same windows do not seem to have any blood on it, the next shot is of the windows again and there's blood all over it.
When Mrs. Lovett is singing "The Worst Pies in London", she goes over and stirs a huge bowl of mixture on the counter. In following shots as she dances back and forth from the counter, the position of the spoon in the bowl changes and the mixture also differs in appearance.
During the beginning of the song 'The Worst Pies in London,' the onions stuck on the knife Mrs. Lovett sticks in the counter are on the right side of the blade, facing away from the door (and the camera view). When she returns to the counter after having Sweeney take a seat, the onions are still on the same side of the blade, but they're facing to the right of the camera view and the amount is noticeably less.
Throughout the film, the bumps on the beggar woman/Lucy's face changes. When she is speaking to Antony, the bumps on her lips are on the left side of her mouth. At the end of the film, when Sweeney is holding her, the bumps are on the right side of her mouth.
When Signor Pirelli is singing during the contest, he makes the sign of the cross, but he does it with his left hand instead of his right, which not usual for a Catholic. There are two explanations for this. One: Pirelli is left handed (see Trivia) and uses this hand indefinitely. Two: Pirelli lies about his entire persona, so he might as well be lying about his religion as well.
In the "by the sea" song fantasy sequence at the beach, Mrs. Lovett is shown wearing sunglasses, which weren't invented for outdoor wear until 1929, and her bathing suit is more of the fashion worn at the end of the 19th century. Sweeney Todd is also wearing a late Victorian suit. The movie is set in 1846. One must remember, though, that this is Mrs. Lovett's fantasy. For all we know, she could've been the first person to conceive these fashions.
Although the actual mechanism of the chair is not shown, the pedal which Sweeney pushes to open the trap door and send the chair back appears to be switching sides of the chair throughout. There could be two pedals, one on each side, that operates the same mechanism.
During "Epiphany" when Sweeney Todd has his razor up to Mrs. Lovett's throat, the razor's edge is facing away from her throat. The razor's angle shows the razor slightly closed, and the edge faces down into the handle.
The goof item below may give away important plot points.
Throughout the film when Todd presses the chair's pedal, the chair slides backwards as the trapdoor opens, sending the victim below. However when Todd kills the beggar woman/Lucy he presses the pedal and the trapdoor opens, but the chair remains still without sliding back.
When Todd is killing Pirelli he cuts his throat above Pirelli's Adams apple just under the chin, but you can clearly see the slice below Pirelli's Adams apple. Furthermore, as the cut is being made, you can see the blood squirting from the lower spot, while the knife is exactly below the chin.