One of Snape's memories show Lily telling baby Harry to be safe, be strong. That had to have occurred before Voldemort killed Lily. Snape was not in the house until after Lily was killed and Voldemort was gone. Only Lily and Harry should have had that memory, but not Snape.
In the scene where Hagrid has dropped a "dead" Harry Potter, look quickly at George. He turns his head to the right and yells "Fred!" Fred has already been killed by Bellatrix Lestrange so it seems he may have forgotten this in a moment of grief.