Are there any illegal positions that are difficult to spot? I have a question and I'm sorry if it sounds very uninformed, I'm not a professional chess player so my interest is mere curiosity. I was wondering if there are examples of illegal positions that require more than a superficial look at the chessboard to be recognized as illegal.

All examples of illegal positions that come to my mind are pretty trivial.


A missing king
Two kings of a color
Too many pieces considering possible promotions (e.g. 4 queens + 6 pawns)
Pawns in the first rank
Pawns in the eight rank
All pawns in the second rank and other pieces in higher ranks (except knights)


It's very easy to see that these examples are all illegal. Are there any illegal positions that require more insights to spot?
Yes - the less a position looks like a real chess game, the harder it is to spot if it is illegal or not. Sometimes, retrograde analysis is needed to prove a position can be reached in a legal way. For an example, see the starting position of the Horse Concoction by Harry Goldsteen, which can be proven to be legal.
[FEN &quot;8/7P/1P5B/2B1Q1n1/3nn2P/1PRnk1nR/3nnnK1/2B1nQBn w - - 0 1&quot;]

However, as White's pawns have to have captured at least five black pieces, the same position with an additional black piece is illegal:
[FEN &quot;8/1b5P/1P5B/2B1Q1n1/3nn2P/1PRnk1nR/3nnnK1/2B1nQBn w - - 0 1&quot;]