Continuity mistake: In the scene when Ada Wong walks into the Umbrella control room and kills all the agents, she stands in the middle of the room. She uses a gun to kill all the agents, who are sitting behind glass computers. In that scene, and in later shots, we can see that the glass is intact and has no bullet holes in it.
Revealing mistake: When Ada Wong kills three officers in the Umbrella Central Control Room, you can see she shot them from the front. They are sitting behind glass panels. Later, we can see the blood splatter sitting on their side of the glass, indicating that they were shot from behind. The glass is also intact, despite a bullet supposedly passing through it.
Continuity mistake: When housewife Alice is trying to escape with her daughter in the laundry room, she slams a mop through the ceiling. When she slams the mophead through the ceiling the last time and pulls it out, the hole left behind is maybe one foot across. When she lifts her daughter up into the hole it is now a good two or three feet across and reaches from ceiling support beam to support beam.