I recently taught my partner to belay using a Grigri indoors, simulating a climb by walking up a flight of stairs clipping the handrail fixtures as bolts and simply pulling on the rope to simulate a lower off. I thought he had it down when we went for our first ourdoor climb together. I rehearsed everything we'd been through before we started and got him to show me how to pay out slack, take it in and lower off before we started. I reiterated the importance of never letting go of the dead rope, no matter what happened, several times. We then practised him catching me on a couple of controlled falls and a couple of lower offs from the first bolt. I put up a top rope on the first warm up route of the day. As he was lowering me off I suddenly flew down several metres of rock. I looked down to see my confused looking belay partner with his left hand on the Grigri and no hand on the brake rope. He'd yanked the lower off lever back too hard and as the rope running through his break hand began to give him a friction burn he just let go.