Commonest and most dangerous mistakes are failing to look and failing to think.
I think palm steering looks retarded and tiring to perform. It also seriously reduces steering control. Apart from the occasional very tight reversing move I never do it.
You see these chavs round here with right palm on wheel; left hand hanging onto gear stick; left foot perched over clutch in anticipation of the next lightning speed gearchange. All in 1.1 litre Saxo with four passengers jammed in.
The most common mistake is believing that you are a better driver than you actually are.
To be good at anything you need both training and practice. All the training most drivers get is just enough to scrape through the test. They then assume they are fully trained (they haven't even driven on a motorway and probably never been out in the dark)
Simply driving around isn't practice, it's looking for somewhere to have a crash