Vinokorov in Le Tour De France two years ago. Had crashed heavily a few rounds into the tour and had to have his knee all bandaged and stitched which, as you can imagine, would be very painful for a cyclist. Despite this set back, not only did he pledge to finish the tour (which still had more than two weeks worth of 180 k per day races left), but he actually managed to start clawing time back. Everyone was thinking the same thing, wow. He was even clean from drugs. what a guy, in pain, and still racing. It all came crumbling down when track (needle) marks were found on his body. Although he wasn't taking drugs, he was having his blood taken out of his body, replacing it with someone else blood, whilst his was being 'treated' to allow him to take more oxygen on board. Who goes to that length to cheat? He was a top cyclist in the first place so had no need to do it.
Amazingly, he is allowed to ride again this year.
Also, Rio's "I forgot about the test" incident was a pretty basic way to cheat getting caught.