Rising (1) - S1-E1
Continuity mistake: When they first arrive on Teyla's planet, the DHD is close to the gate. When the wraith attack, the DHD is much further from the gate.
Continuity mistake: Right at the end, after the entity has gone through the gate, we get an overhead shot of McKay lying on the ground in front of the gate. His legs are stretched out straight. In the next shot, one of his legs is bent. He's unconscious, so he shouldn't have moved.
Continuity mistake: In the beginning when the crew is in the jumper, Ronan Dex has his hair tied back. When they land on the island and walk into the camp, his hair is down, and then they have to run back to the jumper when the prisoners start shooting at them. When they get back in the jumper, Ronan's hair is tied up again.
Continuity mistake: When they finally get through the gate by blowing the rear hatch (which serves as the exit ramp when open), they arrive on the other end with the ramp closed.
Continuity mistake: In the scene where the Wraith attack on the Mainland, Sheppard is stunned and falls with his right hand on his gun near his hip, but when McKay and the other guy grab him, to pull him to the gate, both the gun and his right arm are above his head.
Continuity mistake: In the scene where Michael is first introduced to his quarters, he goes to pick up a picture of his parents. In the first shot, the laptop next to the picture is closed, but in the next shot through the security camera, the laptop is wide open.
Continuity mistake: After returning from the Daedalus, McKay talks to Nola about how he had been a god to them the past years. In the shots filmed from his front, the standard radio headset is visible in his ear. In the shots from behind, it is gone.
Continuity mistake: When the Atlantis team finds out that the experiment has caused problems in another universe, McKay's sister says to Rodney, something like,"You said that the chances of connecting to another universe that was inhabited were astronomically small." It was in fact Carter that said that.
Continuity mistake: When the Wraith have taken over the Daedalus, Dr. Keller's hair changes from being wavy to being completely straight and back to wavy.
Continuity mistake: Right at the beginning, the crew of the jumper only dials five symbols into the DHD before hitting the engage button.
Continuity mistake: Before Merell enters the cage to be fed on by the Wraith, his collar is open. Next shot when he enters the cage the collar is fully buttoned. (00:28:08)
Continuity mistake: Lt. Colonel John Sheppard is packing his belongings up and his skateboard is against the wall at first, then it is on top of boxes.
Continuity mistake: At the end of 2-2, Runner, Weir shows Ronon images of the destruction of his home world - a fact he had been unaware of. But in "Trinity," Ronon says "all these years I thought I was the only survivor."
Continuity mistake: During the episode Ronon has flashbacks and keeps referring to "the gate" when trying to make his wife leave. It was only after Ronon met the team from Atlantis he begun using the term Stargate as most (if not all) prople in Pegasus refer to the Stargate as the ring of the ancestors.
Continuity mistake: In several scenes (mostly on the planet) we see characters reacting to tactical information from the oracle as if it's displayed in real time. However, the units in the display are moving far more quickly than they could in real-time.
Chosen answer: Because the ZPM allowing them to dial Atlantis was being used at the Antarctic defense outpost. They couldn't risk making Earth vulnerable to attack from Anubis by depleting it with repeated dialing to the Pegasus galaxy. Once they knew Atlantis was in serious danger, they had no choice.
Grumpy Scot
ZPM from Antarctic was depleted in first dial, dial in Siege was done via ZPM from Egypt (SG-1: Moebius Part-1/2).