When Alex and David are fighting on the floor David pins Alex to the floor and grabs a knife from the worktop to stab through Alex's shoulder. The knife on the worktop is regular 14 centimeter kitchen knife which would not go through Alex's shoulder. However the knife through Alex's shoulder and the floorboard later on in the same scene is at least 30 centimeters.
When Juliet, David, and Alex are removing the body from their flat, Juliet puts on a pair of latex gloves. The next shot after that, when Juliet pushes the body off of the bed, she is not wearing them.
When Alex calls the inspector, just after he finishes dialing he runs his hands though his hair and the cut on his forehead is missing. The scene cuts away and when it returns the cut has reappeared on his face.
The film is set in Edinburgh and Alex works on North Bridge, presumably for the Evening News newspaper. However, when the first body is found in the woods Alex arrives home with an armful of the copies of the Evening Times, Glasgow's evening newspaper (the movie was partly shot in Glasgow).
In the scene where the trio are driving across North Bridge at night, they have a lengthy conversation before the camera cuts back to the rear of the car, still crossing the bridge. They could have crossed the bridge twice at the speed they are driving.
When the second intruder enters the loft area to get the money from the water tank he reaches around a wooden support to turn the light switch on. The light switch is not visible to him yet he reaches for it as if he has been to the loft before.