Factual error: When Sam breaks her leg, Dr. Keller discusses with McKay the possibility of internal bleeding, then returns to splint her leg and offer her some ibuprofen for pain. A doctor concerned about internal bleeding would never offer a patient ibuprofen, as it inhibits blood clotting.
Deliberate mistake: As the Wraith "stun bomb" activates at the SGC and everyone falls to the ground, it is visible how everyone uses their arms to dampen the fall.
Quarantine - S4-E13
Revealing mistake: When Dr. Zelenka drops into the room from the ceiling vent, the wall with the monitor projecting the map of Atlantis vibrates as he hits the floor. (00:38:00)
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).