After Jonathan arrives at apartment and smells gas he kicks the door open then goes to turn cooker gas off. He then slams oven door closed. This is very dangerous thing to do as you want to avoid making any spark. By kicking oven door closed would result in metal touching metal making at least a spark.
When Blair is pointing the revolver toward the camera just before reaching the stairway landing and shooting Darryl, the gun has no bullets loaded.
Max says he's going to put the Bentley away, but instead of heading to the front door, he turns to his left.
After David poisoned Margaret, he wiped his fingerprints from the wine glass he used. He wore a glove on his right hand, but nothing on his left. After cleaning the glass with his gloved right hand, he put the glass in the cupboard with his non-gloved left hand, again leaving his fingerprints on the glass.