I think there are 2 main types of lopsided trades here:
1.) An established player is traded for crap and draft picks
ex.) The Carter
and Gasol trades comes to mind
2.) A young player with potential is traded for another player/s
ex.) Nash, Nowitzki, Pippen, Kobe trades
I think Type 1 trades may be the worst kind of lopsided trade because you know you are giving up a solid player and getting crap in return making the GM look like a fool.
Type 2 is all dependent on the ability of the young player to reach their potential which can be very difficult to predict. At the time it may have seemed like a good trade, but came back to bite the GM in the ass.
My 2 cents: This is the danger of trading a young player like Bargnani, who years later could blossom on another team.