Roman Abramovich is credited with reinventing Chelsea into the European powerhouse they are today. While the club is notorious for big splurges and sacking many a manager, it has inherited a sense of community and prosperity despite the ruthless pursuit of titles and victory under Roman Abramovich.Mistakes made in Roman's ReignDespite their immense enviable success, there are many mistakes that have been made over the last 18 years of Roman Abramovich's patronage at Chelsea. Here are five of the biggest mistakes that will go down in history.5. Sacking Carlo AncelottiCarlo Ancelotti's tenure at Stamford Bridge proved to be immensely popular with players, fans and management alike. An inspiring father-like figure who has gone on to have arguably the most diversely decorated career of any modern day football manager, Ancelotti was the perfect match for Roman Abramovic's hefty expectations.Ancelotti operated with a sense of trust and faith in his players, with the squad in his two seasons boasting the likes of Didier Drogba, Petr Cech, John Terry, Michael Essien and Michael Ballack, all of whom were not just superstars but national captains.On a tactical front, the attacking 4-3-3 and at times 4-4-2 diamond provided Chelsea with some of the most attractive attacking football Stamford Bridge had seen up to that point, securing a historic double in 2009/10.The unfortunate fallout in the subsequent 2010-11 campaign came on the back of a second-place Premier League finish to Manchester United. An unceremonious Champions League semi-final defeat to the Red Devils led Chelsea to endure a trophyless season. At a time when Roman Abramovich was obsessed with the Champions League trophy, a league title or domestic cup was considered as a minimal par, which Ancelotti fell just short of.4. The Purchase of Fernando TorresChelsea Press Conference to announce new signing Fernando TorresClosely tying with #5, the purchase of Fernando Torres is a controversial addition on this list , in consideration of the Champions League and Europa League titles won in his time at Chelsea. Despite the accolades, the general consensus is that Torres was a huge disappointment at Chelsea and was a poorly timed transfer for a player past his prime. Bought for a jaw-dropping 50 million pounds in January 2011, Torres looked a pale shadow of the clinical and stylish striker who played for the Reds in Liverpool and Spain. Struggling for form fitness, his four-year sojourn at Chelsea saw him lose confidence and question his own ability. Many Chelsea fans saw him as a liability, wasting clear cut chances that his predecessor Didider Drogba thrived on. The hefty price tag also put pressure on managers to select him in the team at a time when the alternative options on the bench seemed better placed, a lesson Roberto Di Matteo harshly learned in his final game in charge where Eden Hazard was deployed as a central striker over Torres, the resulting loss costing him his job.