Kenyan Asbel Kiprop has been given a 4 year ban for doping. Kiprop is the 3rd fastest male 1,500m runner of all-time and many time global champion over the same distance. His story includes admission of monetary payment to a drug tester as well as advanced notice of drug testing, of which there have been other recorded examples, including:

On Twitter, Ross Tucker did a good job of walking through the various defenses presented by Kiprop:

Although a lengthy read, it's worthwhile to work through the Decision of the Disciplinary Tribunal to get a sense of how this process works.