(snip)
Higher than the actual clash rate on the current system?
I have heard stories about NICs produced with the same address, most likely in the same batch with a high probability of being installed on the same net.
We once had a Sun system that the local computer hardware people sent back to Sun to be fixed. The rule at the time was that one should remove the ROM (or battery backed RAM) before sending it in. In this case, it was reinstalled backwards destroying the stored address. Losing the stored address from battery backed RAM can't be that uncommon. (The battery life tends to be less than the processor life.)
Also, I once knew a net with a device with address 00:00:00:00:00:00. As there was only one, it was decided to leave it alone.
-- glen