Happens all the time. Hell last year Ryan Braun, the freaking MVP of the NL, tested positive and got off due to a sampling procedure error in which the tester had to take the sample home for the weekend because Fedex was closed (which is the freaking procedure they are supposed to use). Got off with no suspension. Players are protected by their agreements with the league through the union requiring specific methods be observed and the NFL guy testing Sherm fucked it up simple as that. Who knows if he really tested positive (I'd bet he did), but once the procedure is flawed its over. You'd think they would just come back the next day or later that same day and get another sample though if there was a procedural issue. It isn't like people take pisses once per season.