When Alice and Jack are hugging after she rejects his proposal, Hatter is seen entering in the background. A moment later, there is a shot of Hatter actually entering the building.
When Alice, Hatter, and White Knight are on the roof, the White Knight is sitting on the flamingo on the end. When he flies off, he was on the middle flamingo.
When the Hatter, Alice and the Knight are being chased on the Pink Flamingos The men chasing them are firing twelve gauge pump action shotguns. They never use the slide once to chamber the next round before firing it.
When Hatter charges the suits, the horse of the White knight starts to pee.
(at around 1h 30 mins) Where the White Knight is setting up his army, you can see a yellow Excavator in the background of the field.