A three-dimensional (3D) printer includes a base plate and a nozzle connectable to a source of molten material and operable to eject the molten material onto the base plate that are movable in orthogonal directions to form a 3D object on the base plate. An error detection system detects a dislocation of the 3D object on the base plate during the 3D printing operation, which can lead to stopping the 3D printing process. The error detection system includes a beam emitter and a detector that work in conjunction with a tag formed on the 3D object to determine whether the object has dislocated.