When Cristina draws the line on Claire's leg with a Sharpie, she uses a red Sharpie. When she checks it a little bit later and the infection had grown, the Sharpie on her leg is black.
When Derek is in the closet with Meredith, he gives her a bag to breathe into. The bag has an orange sticker on one side. In one shot, you see the sticker on her right, and in the next shot it disappears. Then it reappears on the her left side.
Izzie reveals that her hometown is Chehalis, Washington. She pronounces it "Cheh-hay-less" and it is actually pronounced "Sheh-hay-less".
When Meredith uses her stethoscope to listen to the old lady's lungs after intubating her, she uses the stethoscope backwards.