When Mike drops the phone down, the receiver is placed in front of the phone, however when he comes back to it again, the cord wire is now wrapped around the telephone and the receiver is behind the phone.
When Mike see him hang himself in the mirror, the watch is on the right hand of his alter-self (the stunt double), whereas Mike wears his watch on his left hand throughout the movie. This was not simply the result of seeing his image reversed in the mirror, his alter-self is actually perpendicular to him when he witnesses the hanging in the mirror.
While Mike is reviewing the room for the first time, he puts down the bottle of cognac on the bedside table, opened. When the radio starts and he looks back from the window, the cork has magically reappeared.
After Mike discovers the turned-down bed (and the neat toilet roll in the bathroom), he switches off the radio. The cognac is perched right on the edge of the table. When he tries figuring out where the maid might be, the cognac has suddenly moved back.
When Mike is walking into the lobby, the overview shot shows the woman with the baby buggy and an Asian lady sitting next to her. As the shot comes to ground level, you see Mike walking past the lady with the baby buggy, but the Asian woman is not there.
When the sprinklers turn on and destroy Mike's laptop, he becomes frustrated and closes the screen completely. As he moves toward the heating duct grate, the laptop is shown to be partially open, and it stays in this configuration until Mike talks to Lily when the room is frosted.
While driving from the first hotel, there is an outer shot of the jeep showing a small, 2 lane road, with trees right next to it. In the next shot, there is an additional lane added to the right side of the road, a car and a truck appear from behind, and now the trees are a lot further from the road it's self.
When Mike settles into the first hotel room, he is careful to set his unlit cigarette in the ashtray so it is balanced, neither end touching anything. When the shot cuts and pulls back, the cigarette is well-leaning into the ashtray.
When the room first becomes enclosed in the brick wall, the three main pictures are their alternate versions. It isn't until later in the movie though that there is a specific sequence showing them change from their original to alternate forms.
The Dolphin Hotel is supposed to be at 2254 Lexington Avenue. Later on in the movie it is mentioned to be at the intersection with 45th Street. 2254 Lex Ave, however, is near the intersection with 84th street.
While Mike Enslin is lying on the floor, his hand goes in and out of the bed comforter repeatedly. However, as this happens with each change of shot and is a very obvious inconsistency, it's almost certainly deliberate and done for thematic effect. Especially as, moments later, he keeps changing between lying down and being propped up in between shots, apparently without moving.
When Mike Enslin is on the ledge outside, the size of the ledge changes. When there is a shot of just his feet, he appears to only be able to fit half of his foot on the ledge at most, however when there are long shots which include Enslin's feet, they are fully on the ledge and are given about an inch of space. However, as the movie's main theme is based around reality and Mike's (and therefore our) perception of it, this may well be deliberate - the ledge really is wide enough for his feet but to his eyes it's far too narrow, thus heightening his fear.
If no electronic gadgets can work in the room how does Mike's laptop work? The room is meant to mess with Enslin's mind, so it would be guessed that letting him use these items are for mere amusement of the room.
When Mike contacts his wife using his laptop, it is clear that he's using Windows XP (blue taskbar with the green Start button), however when the sprinklers go off, the well-known Blue Screen Of Death is of the Windows 98 type: 'Windows' in white on grey background, the rest of the screen being blue, and the error message centered on the screen. The XP BSOD type is simply a little darker blue with white text aligned to the left.
When Mike sees him hang himself with bed sheets in the mirror, it's very obvious that the person (a stunt double), is wearing a harness, as the the sheets are centered several inches behind his neck even when he bounces and comes to a stop. It's also apparent that a bungee was used since the person in the mirror bounces back up about 2 feet.
In the scene where Mike Enslin is lying on the floor in the cold wrapped in the comforter, you can see the bottle of liquor very close to his head. In the next scene when he hears Lily Enslin on his laptop and gets up to try to talk to her, the bottle is much further away.