Half way through my mock race, I started having trouble with the timer (a champ timer from best track).
After rebooting the PC, restarting the software, re-powering the timer, I just could not get the "problem" to go away.
I finally figured it out. When testing with the timer not attached to the track, the little optical transmitters/receivers that sit on the foam pads can sometimes get out of alignment with each other (especially after sitting in a box for 1 year since the last use).
When this happens, the timer will display a "C" on its display for the lane in which optical connectivity is lost. I simply nudged the foam block to put the two back in alignment, and everything started working again.
Gave me a real worry for a while there. Thought I would share in case anybody else stumbles across this sometime down the road.