I GOT IT RIGHT, AFTER ALL! 
As someone once said, "Sometimes, it is better to be lucky than good!"
Alabama @ Hawaii, 2002. Hawaii receiver catches a pass near the sideline and is advancing when he 'jukes' toward the sideline and steps OB nearly the length of his foot. As S, I see this and begin stopping the clock and sounding my whistle, and follow the BC with my eyes, knowing the all-time best H in NCAA history (three national championships) would be marking the spot when I turned back. However, instead, he looks at me from about 20 yards with a "WTF" look on his face and says, "What are you doing?" I said, "He stepped out of bounds." H: "He did?" I said, "Yeah." At that point, I realized that neither of us had a spot. What's a guy to do? All I could do was move to a spot that 'felt' right and put my foot down with confidence, which I did. Now I am awaiting an outcry from the Hawaii faithful, indignant that we were marking the spot incorrectly by at least 5 yards. Nothing. Moments later, still nothing. We set the chains. Still nothing. My H can see the video board and the replay, and he gives me a

on the out of bounds, but we still didn't know about the spot. Game ends with neither of us knowing how badly we had missed the spot.
Hawaii replays their games on local TV beginning around 11:00 pm. So, I stay up to watch for that down. About 1:30 am, OK, here it comes. There he is. He clearly steps out of bounds (at a yard line we can identify on TV). We disappear from view, and when we reappear, setting the chains, the down box is less than 2 feet from the actual OB spot. Whew.
That was my "Better to be lucky than good" moment.
Then I went to bed.