#2 Diego Maradona to Sheffield United (1978)

If you went to scout young footballing talent in Argentina back in 1978, it is unlikely you would not be informed about a 17-year-old baller named Diego. Sure enough, then Sheffield manager Harry Haslam came across the footballing prodigy too. Of course, Diego did not fail to blow Haslam's mind, who wanted to sign him at all costs.
However, the Sheffield board were a little sceptical on that matter and decided that Maradona was a little too expensive. Instead, they went for another Argentine Alejandro Sabella for £160,000 who you might remember from his managerial stint with the Argentine national team in the 2014 World Cup. Sabella wasn't a flop by any means, but let's not compare him with perhaps the greatest footballer of all time. Similarly, the Sheffield board might have made a handy profit on Sabella too, but let's not compare that with perhaps the biggest boardroom blunder of all time.