Canon has released a firmware update for the EF40mm f/2.8 STM Interchangeable Lens for Canon Digital SLR Camera.
Firmware Version 1.2.0 addresses the following phenomenon
1. If pressure is applied to the lens barrel while the lens is mounted to the camera (pressure can be applied even while attaching the lens cap or while carrying the camera with the lens attached in a bag), the autofocusing function of the lens may stop working.
A Canon EOS-1D X, 5D Mark III (at v1.1.3 or later) or a Rebel T4i/650D is needed to install the upgrade. Alternatively, the lens can be sent to Canon Service to have the firmware upgrade installed.