The Met Gala has a history of stepping on the wrong hem. Fashion’s biggest night has racked up its share of scandals over the years, from controversial themes and divisive honorees to red-carpet political statements and reviled billionaire sponsors. The latest fiasco — the unprecedented cancellation of the 2027 John Galliano exhibit one month after its announcement, after mounting pressure from Jewish leaders, city officials and Met donors — is just another reminder that the first Monday in May doesn’t always go according to plan. John Galliano: Horizons (2027) The Costume Institute announced in July that the British designer would be…