This thread will give me cold sweats all day.
I was at the first three suggested; Halifax, Luton and Mansfield, and immediately thought of Bury (0-2) in the League Cup in 72-73 which I've always thought worse than Halifax as we were a better team when we travelled to Gigg Lane.
Then I remembered I was at Barnet (0-1) which was bloody awful even though we went through on aggregate, Lincoln a 1-4 defeat having gone ahead in the first minute, rags 0-5, Wigan at Wembley, Derby 0-4 in 76-77, Forest away (1-4) in the FA Cup (73-74?) and 0-3 at home in the same competition in 2008-09; they were in the second tier for both ties.
Wolves ruined my first trip to Wembley in 1974, and then there's Groclin; a 0-0 draw (but lost on away goals) in a farmer's field that made Halifax look like Wembley.
And 2-3 against the rags after being two up when we could have won the title that day. - And any other defeat against our nauseating neighbours.
Edit: Forgot to mention a game at Bootham Crescent towards the end of 1998.