When the porter comes home with the stolen coat, the third button down (which fell off earlier) is still there until a close-up of him at the door.
When the Hotelportier visits the washroom at the end of the film, he puts his cigar into an ashtray. However, it subsequently disappears, and when the Hotelportier gives the Nachtwächter one of his cigars he retrieves his own from the other side of the room (off-screen).
When the porter loads the trunk on his back in the rain, the rain isn't falling on it. Once he takes a couple of steps, it is.
In the Hotelportier's drunken dream, the rope for holding/raising/lowering the trunk is visible as the trunk falls back into the Hotelportier's grasp.
A hand-cranked camera is reflected in a glass door.