Factual error: Derek tells Meredith to do an MRI on the lady that has a fork stuck in her neck. But an MRI-machine is actually a huge magnet; that's why there's absolutely no metal allowed inside of it. Doing an MRI on someone with a fork stuck in her neck is extremely dangerous.
Continuity mistake: We see a nurse writing down "Yang, C" on the assignments board; note the handwriting. Later, when Burke notices that Christina's name is on the board, the handwriting of "Yang, C" is completely different and other minor writing differences are noticeable too.
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 is in the locker room eating a packet of crisps, everyone comes in and she asks them if they have any decent food. In the next shot you can see she is now holding an opened bar of chocolate. Then in the following shot we actually see Meredith hand her a wrapped bar, which Christina then opens (and the previously opened bar is gone, of course.)
Continuity mistake: When Derek sits in the rocking chair in the room with the premature baby and then Addison starts listing their relationship options, the blanket hanging over the back of the rocking chair constantly changes position, evident by the green stripes on it.