First game that came to mind when I read the OP was one a few years ago when we played Arsenal in the FA Cup at Bramall lane. We put out our regular first team and they played their reserves - and spanked us 6-0! That was a complete performance from them. I think we may have been a couple of divisions apart at the time - but I remember going into the game, hopeful. I remember also feeling a bit let down when I heard Arsenal's team sheet - because they'd got loads of young players in there and I was really wanting to see the big name players.
Those young players included: Theo Walcott and Eduardo.

And those two, in particular, ran riot on the night.
Many years before that, I remember the 0-5 home defeat to Arsenal, the match where Alan Ball, sat on the ball, and John Hope was literally praying to God for divine intervention, because he just absolutely lost it completely. Poor kid - I mean, looking back I was probably like the majority of fans, feeling annoyed at Hope for being so incompetent - because that's what he was on that day. He missed just about everything, shots, crosses, the lot. The
piece de resistance being when he went down, legs wide open, to stop a tame shot and it rolled straight through his legs into the goal. But I look back now and think the lad must have had something going on in his head, some kind of confidence attack - because that just wasn't normal to go to pieces like that.
He looked like the real deal, did Johnny, when he took over from Hodgy - 6' 2", good looking young lad - fantastic reflexes, athletic too. But ultimately, error prone to the point where he lost confidence in his own ability. I was sad to hear of his passing a few years ago - and the poor lad finished his life suffering from Parkinson's Disease as well. Not nice. He leaves us with some good memories though as well. But the 0-5 home defeat to Arsenal wasn't one of them.
