All the usual games that get a mention are worthy candidates, but one that no-one would think of mentioning really did prove costly, a 0-0 draw at Sheffield United in March 76 if my memory is right. We absolutely pissed all over them for the entire game, hit the woodwork at least five times and missed chance after chance. They were rock bottom of the league and there for the taking but we just couldn't get the goal that mattered. It was the only point we dropped in a run of 12 games before the Norwich loss which we won't talk about...
Can't pick just one: Vauxhall Motors is high on my list but maybe not as high as the Milk Cup final loss to Oxford (who were those guys in the hoop shirts anyway) the FA Cup final replay and relegation to the third tier - don't remember what it was called then
