The US punk rockers showed the video clip, which included footage of Princess Diana on her wedding day to Prince Charles, during their song ‘Champion’ – something they have previously done during their US tour.
But fans over here in the UK have been left baffled by the move with some branded it ‘bizarre’ and ‘strange’.
A fan at the Cardiff concert at Motorpoint Arena told The Sun: ’During the gig images of Diana were displayed culminating in a short film chronicling her final journey from the Ritz in Paris, her funeral and even the Queen’s Speech from Buckingham Palace.
‘It’s safe to save revellers were confused by this… when it became clear what was happening some went quiet and started shuffling, looking down at their feet, wishing the next song would come on.
‘It was totally bizarre, more than a little creepy. There seemed to be no explanation for Di’s haunting appearance at the gig. What were they thinking?’
Others voiced their confusion on Twitter.
But some fans stuck up for the band and applauded them for depicting the People’s Princess as a ‘champion’.
Fall Out Boy finished their UK tour, which kicked off on March 27, in London’s O2 Arena on Saturday.