user17457: My sources of opening theory are mostly focuses on things to do in it, and some ideas about why.
What are good sources about things to avoid, and ideas that can't be developed in an opening? 

user14202: This site has some material. Things like don't block your center pawns with your pieces, and don't play into fool's mates, etc.
Then there is a series of beginner-ish books from Pandolfini: Chess Openings: Traps And Zaps
also,
Win in the Opening: Opening Mistakes and How to Punish Them 
by Yakov Neishtadt

17457: <p>My sources of opening theory are mostly focuses on things to do in it, and some ideas about why.</p>

<p>What are good sources about things to avoid, and ideas that can't be developed in an opening? </p>
 14202: <p>This <a href="https://www.chessstrategyonline.com/content/tutorials/how-to-start-a-game-of-chess-opening-mistakes" rel="nofollow noreferrer">site</a> has some material. Things like don't block your center pawns with your pieces, and don't play into fool's mates, etc.</p>

<p>Then there is a series of beginner-ish books from Pandolfini: Chess Openings: Traps And Zaps</p>

<p>also,
Win in the Opening: Opening Mistakes and How to Punish Them 
by Yakov Neishtadt</p>
