I'd like to go back to the point made about understanding the logic of the transfer market. I found that a little amusing because I don't think any subsequent posts have fully demonstrated that.
If you're going to be funny about it, then transfers simply can't be looked at in isolation as they always form part of some wider transfer strategy and then an even wider overall strategy for the team / club. Come the summer, the manager and co. sit down and develop the plan for next season, identify the areas they want to improve upon and the players they'd like to target. To really judge the success or failure of a transfer, you have to look at examine it within that context and then look at how everything panned out.
Now, if you take Babel as an example, then I think rebel's argument holds up. We took a punt on an exciting attacking player who, if successful, would fill a void in our team. It didn't work out, yet we're talking about recouping most of our fee. That's pretty good.
If you look at Keane, well... lets.
In terms of transfer strategy for that summer, we know that Keane cost 20M and we know that impacted on our overall transfer dealings that summer and forced our hand into going for Riera at a bit of a bargain price. He must have been identified as a priority signing because we signed him fast and for big money.
In terms of the footballing strategy, we have no idea and come the time, neither did Rafa. We have since heard mutterings of playing Keane up top with Barry on the left, but that doesn't quite stack up and sounds like bollocks. Either way, it just doesn't make sense.
The net result was that it fucked our transfers for that summer as well as having a knock on effect in later transfer windows and it fucked our squad because we were left short - not only because we sold him but because we missed out on an opportunity to strengthen to the tune of a 20M signing.
So overall, I wouldn't class that as a "mistake" a la Babel, but a shocking display of ineptitude. I would go as far as to say that it one of the worst signings in PL history and really just one of the worst I can think of in general. Clubs will have lost more money on players but when you consider the lack of foresight and the ruinous effect it's had, this is right up there.
Dossena is not quite as bad, but I wouldn't class that a mistake either. It only took a few performances for fans to become very worried about our most expensive defender ever and again, Rafa was no different as he became very very reluctant to play a player that was patently unsuited in every way. Buying a player and him not working out in the mid to long term is a mistake. Buying a player and then finding yourself unable to play them a couple months later - and that not changing till you sell him - is just unacceptable.