An Apparatus for heating plastic preforms, with a transport device which transports the plastic preforms along a predefined transport path, wherein the transport device has a plurality of holding elements for holding the plastic preforms, with at least one heating device which heats the plastic preforms transported by the transport device. The heating device has a movable wall element which is arranged for at least part of the time below the transport path of the plastic performs. The apparatus also includes a detection device which detects an incorrect position and/or a falling of at least one plastic preform in relation to the holding element holding this, as well as a control device which initiates a movement of the wall element on the basis of an incorrect position and/or falling of the at least one plastic preform detected by the detection device.