Inexperienced belayer using a grigri 2 on a DMM boa carabiner. Climber wanted to be lowered a couple of feet after which he was to climb again. During the lowering, the lever of the Grigri moved through the carabiner and got caught, sticking it in the rope freerunning position (It is difficult to reproduce the movement, but it is possible!). Maneuvering the handle back to the correct side of the carabiner was difficult and needed some wiggling and couldn't be done with climbers rope in any way tight. Dealt with by initially standing on the rope to make a brake, then another climber putting a belay using an ATC onto the brake rope, so that the rope could be paid out using the ATC through the inoperable grigri.