When the SUV of teenagers crashed, the paramedic says there were eight of them, but in the sequence that shows the deaths of all but one, there are only seven total, including the one who survives.
When Alex is sitting on the bench outside when Meredith approaches him about marrying Izzie, he is shaved. When he comes in Izzie's room in the groom's outfit, he isn't shaved yet.
When the first woman from the crashed SUV is brought into the ER, the blanket covering her changes position between shots, from covering her waist to covering her chest.
When Meredith talks to Alex outside of the hospital he is clean-shaven, but later on at the wedding he has his usual stubble.