There was a discussion here during one of the recent Boston Marathons about the finish line. These big marathons always have multiple timing mats. I don't know if the second one is a backup in case someone's chip doesn't register at the first one or not. I ran through both of them at my last Boston.
Look at the photos below. The first photo is Boston: is the finish line the start of the painted area like it is on the track or in a football game end zone (no timing mat there) or is it at the end of the painted area (timing mat)? The second timing mat is visible where the runner is kneeling.
The second photo is NYC. It seems a bit clearer with the timing mat in the center of the painted area, but I'm sure some people were confused by the multi-colored lines just before the mat, especially after running 26 miles. And, of course, we have the second timing mat here, too.


