It strikes me, here in my little corner of the internet, that the "too many mistakes" problem and the "not enough mistakes" problem could be seen as manifestations of exactly the same problem: suborning character integrity to plot interests.
Most of the really infuriating character mistakes I can think of off the top of my head come down to Plot Necessitated Stupidity: characters only make a mistake when the plot requires that they - say - decide to implicitly trust a complete stranger who talks like Dr Evil.
Similarly most infuriating not-mistakes come from either characters suddenly "knowing" what the right course of action is or - worse - characters doing something irredeemably stupid and having it miraculously turn out to be the right idea all along.
-- Dan Hemmens
(Posted as Morcar on Fictionpress coupla years back, just caught up with your Fantasy Rants after a long absence)
I'd agree that in many cases, the author has too much of an eye on the plot. With absolutely perfect characters, though, or characters that are mouthpieces for the author's POV, the slavish love of the character involved makes them dash to the rescue.
(And hello. I think I remember you!)