When Holly Martins first encounters Harry Lime across the street in a darkened doorway, he doesn't know who it is and he yells out, "What kind of a spy do you think you are satchel foot?" When he says this line, you can clearly see that he is not saying anything at all.
In the sewer, before putting his fingers through the grate, Harry Lime holds the stair's supporting pole with his right hand, but the gun should be in his hand, as displayed before and after this shot.
Harry Lime's sewer scenes were shot in two locations--at a studio in the UK and on location underground in the Vienna sewer system. In the Vienna sewer location scenes you can see Harry's breath (it was cold down there); in the sewer scenes shot in the UK studio you can't.
When Calloway and the police stand waiting for Harry to arrive at the station, an elderly balloon man tries to sell them a balloon. His balloons have designs printed on them. Calloway has his sergeant buy one to get rid of the old man, but the balloon has no printing on it, as the shot of the police was filmed later in London.
The policemen who come to arrest Anna change between shots. In dialogue scenes the British policeman is played by Geoffrey Keen. In shots of the Jeep driving to the apartment, and in long shots, a different actor is used.
In two instances, shadows caused by back lighting don't match the scene. When the little boy is following Holly and Anna you see his shadow on the wall, and then it is on the street simultaneously when he rounds the corner. The shadows are made by two separate persons. Later, when the old balloon man appears, you see his shadow on the buildings, but the shadow shows no balloons. However, when the old man appears from around the corner his balloons show definite shadows on the pavement.
When Holly and Anna hastily leave the scene of the death of the porter, they are at the site of Harry's Lime's apartment in Josefplatz. They turn the corner and are next seen descending some damaged steps with St Rupert's Church in the background. These two locations are a kilometer apart, quite a distance for a just couple of seconds of screen time.
When Anna is taken into custody in Lime's apartment, she is sleeping with the large bedroom windows wide open, even though the scene is set in the middle of winter. The police that come to arrest her are all wearing heavy winter coats, indicating very cold weather.
As Martins arrives at the funeral, he asks Maj. Calloway who the funeral is for. A black tombstone is seen behind Martins. When he walks towards the grave and stands by Anna, the same tombstone is seen behind her. This occurred because both of those scenes were not filmed on location in Vienna, but later at Shepperton Studios, which only had very few fake tombstones available.