The second time Ted goes to see the Once-ler, he accelerates his scooter using the left handle; in the rest of the movie, the accelerator is in the (usual) right-hand handle.
When the two hired goons interrupt O'Hare's meeting, one shows him live footage on his cell phone of Ted driving through the door to the ramp that leaves the city. A moment later, Ted is shown poking his head through that same door and preparing to drive through it.
When Ted's mother escapes in her car, you can see the doll in the back seat. After that it disappears during the whole chase sequence, then appears again when she stops.
Just before the "How bad can I be?" song a close-up of the guitar the Once-ler is playing shows a considerable amount of white tape wrapped round the fret board. Moments later when he is playing the guitar outside, the tape is gone.
When the young once-ler is throwing things out of his wagon at the animals behind him, the big bear is seen in two shots. Then, when the camera widens to see the angry animals, the big bear is walking up and is surprised to see his friends so angry.
Ted's calendar in his room shows 35 days in October.
When the stadium-like lights come on to bring the "brand new dawn", the sky turns blue. This does not work in real life; light can create this effect only if it comes from above the atmosphere.
When the Once-ler gives the last Truffula seed to Ted, it appears as a large pit similar to what is found inside a peach or nectarine; but when animals eat the fruit earlier in the film there is no evidence of the fruit containing such a large seed.
The advertising executive claims that O'Hare Bottled Air comes in plastic bottles, but when the actors in the commercial toast each other, the bottles clink as if they were made of glass.
During the "Let it Grow" song, once Ted's family and Audrey start singing, you can see O'Hare's employees' mouths moving but you can't hear them singing. They might just be lip-syncing.
The film never explains about how does Grammy Norma know so much about the Once-ler. The film never showed that she had a relationship with him, or even shown in the Once-ler flashback, with Norma as young as the Once-ler was. Yet film shows that she knows where the Once-ler lives, and even mentions about the fact he exist to Ted without any explanatory reason.
When the delivery man is shown making his first delivery, his vehicle is shown driving on the left side of the road. However, in all other street scenes it is clearly shown that all other vehicles drive on the right.