Continuity mistake: The scene where the bomb explodes shows the bomb squad captain walking toward the end of a hallway. Just as the bomb explodes you can see he is about 5-6 feet away from the end of the hallway where another bomb squad tech is waiting. After the explosion the scene cuts to Meredith flying backwards then to a shot of windows being blown out then back to a rear shot of Meredith hitting the floor. Notice here that a second floor is not visible at the end of the hallway. The scene finishes with a final aftermath shot of the hallway as the debris settles. Here we see an entirely different hallway with a visible second floor and the end of the hallway extended by about 10 feet.
Continuity mistake: When Christina notices the bomb squad moving Meredith down the hallway with the gurney, she leaves the room and removes her scrub jacket and hands it to a nurse. When she is out in the hall and reaches them, she suddenly has the scrub jacket back on again.
Character mistake: Meredith states that she cannot remember the last time her and Derek kissed. Derek says that the last time was when they were still dating, but it was actually at the end of season two episode three, after Addison arrived in Seattle and Derek and Meredith had stopped seeing each other.