Martin Rickett/PA Wire/Press Association ImagesFor a side that has won 38 trophies, including 13 Premier League titles and two UEFA Champions League crowns over the course of the last three decades, you might not think Manchester United have made too many mistakes in recent history. Regarded as one of English footballs greatest ever club sides, the Red Devils have conquered all before them to sweep up an unprecedented haul of silverware, much to the delight of their legions of fans. But things havent always been rosy for the Reds, and there have been times when things have gone awry perhaps never more so than last season, with the club finishing seventh and failing to qualify for European football. There have been other examples of similarly-costly mistakes throughout Uniteds history, from red card madness in the heat of battle through to expensive transfer blunders and even a flying boot. Yes, Manchester United are by no means perfect, and they have had their share of negative press down the years. Some of these mistakes have been papered over by the clubs recent successes domestically and abroad, while others have had a longer lasting, further reaching effect. With that in mind, here we take a look at Manchester Uniteds 10 most costly mistakes.