Theory in Non-Theoretical Lines
A lot of players I know (even experts and masters) make a fairly significant mistake. In an effort to avoid... well... effort, these players opt for less common openings, hoping to avoid theory. The Scotch and the Scandinavian are the two most popular of these (in my opinion).
The problem with these older openings is that, even though there is less theory, the theory that can be found becomes indescribably more important... a lot of players are willing to take their chances when it comes to studying theory at all.
Here's a fun miniature I played here @ chess.com. It's a great example of why the 'book moves' are worth learning.