Continuity mistake: Just before George withdraws some blood from Bex's arm, her right sleeve is rolled up when Addison is beside her talking. In the next shot as Bex is drawing, the sleeve is suddenly rolled back down and George pulls it back up again to get some blood.
Continuity mistake: When Alex and George are trying to get out of the elevator that they are stuck in, watch the monitor screen attached to the gunshot victim. The lines on the monitor keep instantly changing appearance (from straight, to jagged, and then back, etc.), especially the middle blue line. (00:13:05)
Continuity mistake: When Meredith, Christina and Izzy are standing by the hospital hall balcony talking, the nurse, doctor and patient extras walking and passing in the background are inconsistent from shot to shot. In addition, the position of the three ladies arms / hands on the banister keeps changing during this scene. (00:12:35)
Other mistake: The exact same shot of Derek's radio tuner is used twice-once when he calls Amelia and again when he gets in the car after helping at the accident. Both times, the clock says 7:45PM, but Alana (the AP class taking, cello playing student) says that she's supposed to be at school, implying it's sometime between 8AM-ish and 3PM-ish. Also, the sun sets around 7:30PM in Seattle on March 26, but it is obviously day time for the duration of this scene so there's no way it could be 7:45PM in either of the shots in which the clock is shown. (00:03:30 - 00:18:40)
Visible crew/equipment: In the NICU, when George starts boasting that he has the best quint, in the overhead wideshot we can see the T-mark (greenish yellow) on the floor near Kate, moments before Karev walks in and stands there. It's visible again just as Karev walks away. (00:01:05)
Continuity mistake: When Alex is giving Izzy her medication, he is holding a water bottle and an unpeeled banana In his right hand. Then the banana magically disappears and he is holding only the water bottle. Immediately, the banana reappears and it is now peeled. This all takes place in three quick shot which are from different takes.
Factual error: Derek performs CPR on Meredith in the back of the ambulance, but he does so incorrectly. He gives one breath and five compressions instead of two and thirty. Also, drowning victims need oxygen, and Derek should have used a BVM to ventilate Meredith, not mouth to mouth. Any doctor would have known how to provide proper treatment.