Other mistake: Before the battle, the leader of the waterbenders says that he will have everyone extinguish as many sources of fire as possible. When the firebenders come up from the ground at the start of the battle, all of the torches lighting the city are lit.
Other mistake: When Aang creates the ice spear things during the battle, he throws them into the ground surrounding a few firebenders. The firebenders freeze as if frozen entirely in ice, despite being only caged in by ice spikes.
Revealing mistake: During the first conversation between Fire Lord Ozai and General Zhao we can see that Ozai's eyes move from left to right while he looks in the direction of Zhao's chest, this revealing the use of a prompter.