Mistakes Not Created Equal

When I listen to someone play and they make a mistake, what is the cause of that mistake?

Is it memory?

Or is it lack of understanding?

Lack of technical mastery?

When I am judging a piano competition and a pianist messes up a spot invariably it is in a harmonically complex part. At any point in the piece you should know what key you are in and what chord you are on.

When students see the notes they play as a collection of random notes it tends to lead to certain mistakes than someone who understands the harmony would never make.

This reminds me of the chess study.