The keyed washer seen on many threaded headset was never intended to act as a "lock washer". The key way system was intended to keep the lower adjustable race from turning as the top locknut was tightened. In theory, this means you can use only one wrench, and not hold the lower race. In practice, it is necessary to use two wrenches, one to tighten the top nut and a second to hold the lower race. If fact, you can find many threaded bikes where the keyed washer has rotated into the thread, cutting and damaging them. The keyed tab is effectively a "tooth", doing far more damage than good.
Once again, we see it is tension in the threads that holds the fastener system secure. Get it tight.
