Continuity mistake: After Mortimer picks up Elaine and literally throws her out the front door, he locks it to make sure she doesn't just open the door and walk in again. When Aunt Abby goes to open the door for Mr. Gibbs less than a minute later, she doesn't have to unlock it.
Continuity mistake: When Mortimer discovers the body in the window seat his hair changes from being flipped over his forehead to being neat and then back again without any obvious motions that would have changed it.