What are the common "non-blunder" mistakes non-beginners make?

A week or so ago back in post #76 I asked about a principle I thought I recalled that advised not to pin Black's Knight on f6 before Black has castled.  I didn't recall where I had seen it, or the logic behind the principle.  By happenstance, I just came across this in Dan Heisman's June 2005 Novice Nook column titled Opening Principles:

Note that the diagram is not correct for the position after 12...Nd4 -- what happened to the Bishop on c4 , and why is the Bishop that initiated all this trouble still shown on c1? 

Heisman's column wouldn't have been the original source for me, but since Heisman identifies this as one of Lasker's rules, it probably shows up lots of places and I could have seen it anywhere.  And if the principle applies more to 1.e4 e5 games, as Heisman says, I picked a poor example (French) in the original post when I asked what the problems might be in going against this guideline.

--Cystem 

Being a weak noobie is a common mistake..

playing