While 11.19/11.20 take place through the course of two days,
11.21 starts at the beginning of #11.19 and surpasses the plot of 11.20 is said to take place within 24h. It's still the day of the accident when Meredith arrives at the hospital.
When Charlie's wrecked car is being hauled away, upside-down on the flat-bed tow truck, all the tires are seen to be intact. The car was immolated by an un-controlled fire, which burned for at least 10-15 minutes. The tires would have melted or exploded from being exposed to such extreme heat for so long.
When the nurse is shutting off Dereck's infusion pumps, all 4 of them are labeled as if they were delivering the same drug: gentamicin (an antibiotic).
Derek was trying to answer to his mobile while in the car, but lost it and then it stops while doing a U-turn, and this caused the car accident.
But Derek was driving a Porsche Cayenne, a car which has a Bluetooth communications, and in fact he talk hands free all the time while in the car.
So it remains unclear why he needs to get the phone to answer, while he have a easy command on the wheel and on the radio...
At the end of 11.20, Meredith is sitting at home at 5 p.m. Outside is dark, suggesting the season is winter. Yet, on the same day of 11.21, Derek's car shows his accident occurs at about 7:45 p.m. (due to the time display on the information panel of his car). There's plenty of light left outside, suggesting the season is late spring or early summer. Thus, there's a notable disparity between the situation of the two characters. Also, why would Derek be at the accident all day? He goes to the airport and takes a detour due to the traffic in the morning. Moreover, the date displayed on the car's panel of late March seems incorrect for the plentiful amount of daylight left.
S11 E21
Derek is driving in a certain direction. In front of him there is a car crash. After helping the victims, he turns the car in the opposite direction and gets hit by a car running from the opposite direction. Logically, there was no need for him to turn back.
Derek is speaking on the phone via hands free connection before the accident - there was no need for him to stop and retrieve his phone while making the Y turn to follow the ambulances to a hospital (that did not have trauma center).
Derek cannot cal 9-1-1 because there is no cell phone service, yet he is hit by the truck because his phone rings, indicating there was cell service.
There is a half mile between where Derek is stopped and the curve, and yet a truck running up hill could not see a stopped SUV and stop.