Upload & Sell: Off
My first 1Dx came back without the mark; in the rush to do overnight turnaround, they had forgotten to put it on. However, it ended up having to go back again, as it was unable to track lateral motion. That time they replaced the mirror box. Oh, and also put the mark on it.
The missing mark was just an oversight. I wasn't terribly concerned since I had the paperwork showing it had been serviced. OTOH, the inability to track lateral motion was a serious issue.