If pressure is applied to the lens barrel while the lens is mounted to the camera the autofocusing function of the lens may stop working. Attaching the lens cap or carrying the camera with the lens attached in a bag can cause the error.
The problem does not occur during normal shooting and does not result in a malfunction of the lens. To restore autofocus, detach and reattach the lens, or remove and reinsert the camera battery.
Tests suggest that not every EF 40mm lens is affected.
Firmware to address this phenomenon will be available for download in late August, 2012.