I never hear anyone being booed for misplacing a pass or the like.
What you will often hear however, is a crescendo of noise at those moments, in the form of thousands of groans, and a few choice expletives. If you were that way inclined you could view it as encouragement, but wherever you stand, it isn't booing.
There's no mistaking booing. Booing is a great big fuck off, there is no misinterpretation.
The only player I can recall actually being booed recently, is Kolarov. And that was after he responded to the above frustrated crowd unrest by fronting up people in the crowd and swearing at them during the last game of our relatively shite last season. It wasn't the first time he'd done it either, but after the general unrest and discontent of the dreadful showing in the FA Cup final and Mancini's subsequent sacking, his behaviour was a lighter fluid to a flame.
Just thought that distinction was important.
But yes, the hostility doesn't help the players when they're playing poorly.