Deliberate mistake: When the crow drops Shelley's ring into Sarah's hand at the end of the film, the sound of metal upon metal is heard. (01:30:00)
Deliberate mistake: When Eric first enters his old apartment, watch very closely. When he first wanders in the room, his body has a slightly weird jitter and blur to it that doesn't quite match his movements. This is because the footage of Eric walking through the doorway is actually taken from the scene in the alley. They "cut" Brandon Lee out of that scene and inserted him into the apartment since they didn't have a shot of him. But unfortunately, it has some odd "glitches" due the footage not quite matching.
Answer: There is no explaination given in the films, they are just brought back when the crow feels it is the right time to avenge their murder.