Saw this on the roadraceautox forum and thought I'd share...
"Make sure any cones at/near the start/finish line are spray painted a flat color. Seriously.
After what felt like YYEEAARRs of battling false trip issues, one of Houston's T&S folks spent a couple hours on the phone with Race America about their equipment. When time, sun position, running conditions, and so on were discussed it was theorized/discovered that the Race America (and I imagine others') timing lights are sensitive to reflected light and/or any dust/debris that changes that light. Houston's equipment guy plasti-dipped a couple of cones for the next event and we have rarely had problems with false trips since then."