The Boy in the Iceberg - S1-E1
Continuity mistake: In this episode, Katara remarks that "This ship has haunted my tribe since Gran Gran was a little girl. " However, in the episode "The Puppetmaster" it is revealed the ship wasn't beached there until Gran Gran was in her late teens or early twenties.
Suggested correction: If Gran Gran was really in her late teens or early twenties during the Southern Water Tribe raids, that can still be considered a "little girl," since she is in her eighties in the show.
Suggested correction: You sure that was the same ship, and what's 5-10 years give or take compared to Gran-Gran's life anyway?

Continuity mistake: When Aang emerges from the water on the giant sprout in The Avatar State, the arrow on his head is not glowing.
The Southern Air Temple - S1-E3
Continuity mistake: When Gyatso is laughing in Aang's flashback, he is missing quite a few teeth. But when Aang finds his skeleton, all his teeth are still in place.
Imprisoned - S1-E6
Continuity mistake: When the gang meets Haru, Katara teleports from behind a log (shoulder still visible) in one shot to greeting Haru several yards in front of the log a split second later. (00:02:50)
Suggested correction: That's because the second shot is a jump cut. The trio first sees Haru from behind the log, and after Sokka says, "He looks dangerous, so we better approach cautiously," the scene jumps forward in time to Katara greeting Haru further away from the log. Katara simply got closer to Haru, which was not shown.

Continuity mistake: After Meng gives Aang the tray of food and Sokka takes it from him, Aunt Wu shows up, and it shows a shot facing her front. In this shot, Katara's back braid is missing; then it reappears a moment later. (00:07:05)
The Northern Air Temple - S1-E17
Continuity mistake: When Aang, Teo and Katara are walking up to the door of the Air Temple Sanctuary, the pipes which make it open are in a horizontal position, but in the next shot, they are vertical. (00:09:20)
Continuity mistake: When Zuko and Iroh are walking towards Azula's ship, the gold flame insignia on the imperial firebenders' uniforms switches directions between shots.
Avatar Day - S2-E5
Continuity mistake: When we first see Aang in the stocks, Sokka advises him to use his Airbending to escape and begins "imitating" Aang's kata. During one pose, we see Sokka is wearing the boomerang, or at least the sheath it was in, that he had to leave behind at the beginning of the episode. He wasn't wearing it before, isn't wearing it after, and doesn't actually get it back until the climax. (00:02:09 - 00:06:33)
Avatar Day - S2-E5
Continuity mistake: When the village leader switches the wheel of punishment to Community Service, it is on the space next to Boiled in Oil. However, all of the other shots of the wheel have another section in between these two.
Bitter Work - S2-E9
Continuity mistake: When Iroh draws the symbols of the Four Nations in the sand, there is grass on the left side of the Earth symbol in the close ups. In the shots that show all four symbols, the grass is gone. (00:14:25)

Continuity mistake: When Katara is trying to heal the wounded earthbender, she gives Sokka the idea to take down The Drill from the inside. Sokka then stops her from saying anything while she is sitting down, and she is wearing her necklace. When she stands up in that same shot, the necklace is gone, like they forgot to animate it. (00:07:35)
Continuity mistake: When Ghashiun orders the sandbenders to ransack Appa's saddle, Appa's eyes are closed. In the next shot, they are open, then go back to closed in the following shot. (00:02:23)
Continuity mistake: When Azula is asking Zuko whether or not he thinks the avatar may have survived, Zuko goes into a flashback. When he snaps back to real time, Azula has her hands behind her back. When the shot cuts, her hands are instantly by her side.
Continuity mistake: Toph mentions the worst thing about being disguised is they don't get the hero worship anymore and she misses the love, to which Sokka grumbles, "Boo-hoo, poor heroes." Then, as Sokka says, "It's just, all you guys can do this awesome bending stuff," the triangular emblem on Aang's red "headband" has vanished, and in the next shot, it reappears. (00:03:35)
Continuity mistake: When Piandao tells Sokka that creativity, versatility, and intelligence define a great swordsman, he holds out Sokka's sheathed sword with its hilt at Piandao's right hand. However, in the next shot facing Sokka, the sword in its scabbard now lies on Piandao's hands the opposite way, with its hilt at Piandao's left hand. The position of Piandao's hands and thumbs changes as well, between shots. (00:17:25)
Continuity mistake: In the shot after Mai says "Saving the jerk who dumped me" you can see the rods which the prison guards used to slow down the mechanism of the gondola are nowhere near the wheel. (00:19:55)
Continuity mistake: When Azula says: "What is she doing?" Ty Lee is standing behind her. When the shot cuts and Ty Lee shrugs, they are standing side by side,. (00:20:30)

Sozin's Comet (1): The Phoenix King - S3-E18
Continuity mistake: When Aang is waking up on the giant lion turtle, he is not wearing shoes in the first shot, but in the next shot, he is. (00:22:35)
Sozin's Comet (4): Avatar Aang - S3-E21
Continuity mistake: Throughout the entire show, Aang's eyes are grey, but in the last three episodes (Sozin's Comet Part 2, 3 and 4) they are suddenly brown.