Years ago, the shop I was wrenching at took in a bike to be tuned up that had worn brake pads and horribly adjusted brakes; both levers bottomed on the bars would slow you about as much as sitting up and using your jacket as a drogue chute. We did the full tune up, and restored full function to the brakes. We were joking that we should warn the rider about them when he picked it up. When he did pick it up, I was at lunch. I was walking back to the shop and saw him riding away. He was sitting up, going just above walking speed when someone stepped in front of him. He grabbed a big handful of front brake and did a perfect endo. I kept walking and pretended I hadn't seen a thing. Since then, I have had occasion to warn customers about their properly adjusted brakes quite a few times.