We're showing Houston's Gary Kubiak and Jacksonville's Jack Del Rio as the only NFL coaches without a failed replay challenge this season. The key is knowing when NOT to challenge. Kubiak and Del Rio have challenged only three calls between them. Compare that to Pittsburgh's Bill Cowher and New Orleans' Sean Payton with six challenges apiece. I do think losing teams might be apt to make some frivolous challenges (in the spirit of "What do we have to lose?"). Not sure if the stats back that up over the years, but it makes sense to me. The full coach-by-coach, team-by-team challenge breakdown is below.