People talk about Sam Bowie over Michael Jordan as the biggest draft day mistake in history but for my money Sihugo Green was a much bigger mistake.
In the 1956 NBA draft, the Rochester Royals had the first pick and the Boston Celtics had the second pick (via a trade with St. Louis). With the first pick Rochester selected 6'2" Sihugo Green of Duquesne. With the second pick, the Boston Celtics selected Bill Russell of San Francisco.
The rest is history.
In case you never heard of Sihugo - he ended up playing 9 season (his last with the Celtics BTW) and averaged 9.2 points per game and 3.3 assists for his career.