Ben Johnson

This Jamaican-born Canadian sprinter was the world's fastest man in the 1980s. After winning two bronze medals at the 1984 Olympics, he was a shoo-in for a 100m gold at the 1988 Seoul Games. On the day of the event, he did exceptionally well and clocked a then-world record of 9.79 seconds. 

Unfortunately, a urine test revealed unusually high levels of the banned drug stanozolol and Johnson was stripped of his gold medal just three days after winning it. He was also banned from all track and field events for two years thereafter, scripting a tame end to and otherwise stellar career.