Not read all the posts, but first one that came to mind was Vincent Pericard. Main thing I remember about him is he used to sit on the bench sucking sweet lollipops.
I'm old enough to remember the signing of Chris Guthrie - who I'd suggest held the title of 'Biggest flop in a Blades shirt' for years. Some of that probably was attached to the size of the fee though and the level of expectation that was set - I think he may have been our first £100k signing? (Someone will correct me no doubt if I'm wrong).
Viv Busby was another one that I had hoped for big things from. I think I had him on the bubbly gum cards, so I was dead pleased when we signed him. Thought he was going to be a good 'un. But his appearance in a United shirt only served to give me life-long doubts about any parent that calls their son "Vivian".
More recently, Geoff Horsfield. I have to say that whenever he played against us for Brum he was always a handful. I know he was getting on a bit when we signed him, but I thought he was going to be really good for us. For whatever reasons, he wasn't. Or rather, he wasn't given a fair chance to be good for us. Warnock will have his reasons no doubt - but he was one of a zillion strikers that Warnock seemed to collect, like I used to collect bubbly gum cards, for swaps.