Brian, was this your TL? My old one did it a couple of times.
The question is why did it occur?
1.Improper key or key insertion. I ordered keys several times from Honda, never fit right. Ended up making a key fit EXACTLY right from a larger one. They will push back like Charlie says and prevent a good seat, especially if they are slightly too tall.
2. Improper mating surfaces. If it's sheared before, likely the surface is buggered now from the spinning with the broken key in there. Use a file to knock down any ridges, especially at the keyway, then lap with compound until all shiney.
3. Inadaquate torque when tightening, I'm sure you are using a good air wrench right? I could never hold one and tighten it right with a socket or spanner or torque wrench.
4. This one's just my theory. Timing is too much advanced and/or compression too high. That was my problem after I built the TL to 168cc. I had to run VP C4 (110 octane) to keep the timing advanced and keep it from knocking. Lugging around followed by a burst of wrist and BAM, she'd knock and shear a key. Happened twice. The rotor has so much mass that when it knocks the rotor shears itself from the momentum. If this is happening, repeat items 1, 2 and 3. Worked for me.
Frank