I covered an oldies dance on Hollywood Beach for practice in flash photography. http://www.pbase.com/lautermilch/hbmonday D70, 70-210 P/P & SB800 Anyway, I noticed some shots were dark as if the flash had not fired. I then checked each shot and found it was happening for many shots. I noticed the dance area had traffic cones with rope to mark it off. On these cones were highly reflective tape that were feeding bad information back to the camera. Avoiding the cones fixed the problem.