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.
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: At the very start of the episode when it shows Burke laying on the floor after being shot, there is a very large pool of blood with his glasses right next to the blood. But in the next shot there is no pool of blood anymore but it returns in the next. Additionally, the pair of glasses change position.
Continuity mistake: Cristina walks in during the party with two containers of cupcakes, places them on the side table and walks out. When Alex opens the lid and starts picking at the cupcake toppings, in the shots facing him the top container lid is wide open, but in shots facing Meredith the lid is closed and the cupcakes are untouched.
Continuity mistake: When Izzie is looking at the patient board, there are some lines on it which have been wiped out. But in the next scene we actually see Meredith walk to the board and wipe these exact same lines off (even though they were previously rubbed out...)