Continuity mistake: When Ted visits the Once-ler for the second time and dives to the ground to avoid being launched by the sledgehammer as happened the first time, he stands up and his goggles are around his neck but hanging behind him. The scene then switches to the Once-ler and then back to Ted, and his goggles are still around his neck but now hanging in front of him.
Other mistake: After the song "How Bad Can I Be?", the camera is looking at the animals with 5 trees and the tree cutting machine behind them. As the machine reaches the 4th tree, the tree doesn't fall. (00:56:40)
Continuity mistake: When Ted is playing a board game with his family, his grandmother's letters in her tray change without her touching them. We first see them from her perspective as "LORXBR-". The last one is cut off, but you can see that there is a letter ball there. Two shots later, the O is missing. She now has "L_RXBRA". The grandmother and the letters are visible in each shot, so she cannot have picked it up without us seeing.
Other mistake: In the opening song, at the end when the people are "thhhhhhhhhneeding" it shows a closeup of a little girl with red hair and a yellow bow, you later see her in the flashbacks multiple times. She is throwing tomatoes at the Once-ler and singing along to the Thneed song. There are also 2 O-Hare workers, most noticeably during the opening song. They sing the part about the tree needing 96 batteries. You later see them in a flash back during the "Everybody Needs a Thneed" song. You see them well, in the front row near the end of the song, O-Hare uniforms and all.
Continuity mistake: When O'Hare is telling everyone about how bad leaves are, right after he says "They just fall wherever they want!", there is a mustached guy with a purple shirt with no glasses. When the girl is talking about the trees producing fresh air, however, he now has glasses.
Continuity mistake: During the song 'Let it grow', a woman's clothes frequently change colours. She starts with a dark and mint green shirt and blue jeans. When everyone is going "Aw!" when the 3-year-old is singing, the woman's shirt is brown and wheat coloured while her leggings are white. When the short man is singing, "Let it die, let it die..." the woman now has her green shirt and blue jeans back. Then, in the shot with the line "Plant a seed inside the earth!" (after the line, "Let the love inside you show!") she is visible in the background behind the worker and a man in a jacket and tie, swaying in an orange and red shirt along with burgundy jeans.
Continuity mistake: During "Let it Grow", when "You Greedy Dirt-Bag" is sung, Granny is not visible next to Mary (she is quite far away). In the next shot, she is right next to Mary.
Continuity mistake: During "Let it Grow", in "Let it die..." The short man strides while clapping to the centre of the crowd. After that, he is right next to Mary and his worker who were behind him and to the right.
Continuity mistake: During the 'Let it grow' swarm, the guards are blocked by the crowd, leaving no passage. In the next shot, the guards are all of a sudden closer than they could've got earlier. In fact, during the finger thing, the nearest guard is blocked.
Continuity mistake: When we see a close-up of the Once-ler's guitar just before How Bad Can I Be? his guitar's neck is cracked and taped. In the shots immediately following, his guitar shows no sign of damage and the tape is gone.
Continuity mistake: The wooden door on the back of the Once-ler's wagon changes sizes. It goes from small to large.
Other mistake: While O'Hare was telling people about the cons of trees, he mentioned how trees drop leaves and drop them everywhere, but those trees do not have leaves.
Audio problem: When they are showing the commercial for bottled air the guys clink their bottles together as if they were glass, but after the commercial the ones who made the commercial suggested that their research shows if you put something in a plastic bottle, people will buy it. (00:11:00)
Continuity mistake: In the 'Let it Grow' swarm bit, a guard goes straight from blocked behind and to the left of O'Hare to right next to him.