I lost the exhaust valve in the #6. It cracked in half, and I'm pretty sure it was my fault.
When I was first putting it together, I misjudged TDC on the cam and was slightly off. (I didn't understand how that alignment mark worked up by the cam sprocket) I put in 2 center head bolts to do the timing chain and heard something pop. I took it off and inspected everything, but didn't see any damage, so I just thought it was the head settling on one of the two centering rings, but I think I cracked the valve stem, and it gave out when I tried to crank it.
So, it looks like I'll be ordering a new piston and an intake and exhaust valve for that cylinder. At least its not too major, other than the head rebuild again.