Hacker News Reader

A legal position is any position reachable from the starting position by a valid game of chess, i.e. a sequence of legal moves.

Thus there are no such positions. Every legal position can be arrived at by playing a legal move from another legal position. That includes the starting position, since Nf3 Nf6 Ng1 Ng8 cycles back to it.

I guess what the question is, is whether there's a chess position where every piece is in a legal position, but could never be achieved in practice.

I can think of several, for example if you had all 32 pieces on the board, but pawns were behind each other on the same file.

That's not a legal position, since it cannot be reached in a valid game of chess. What do you mean by a piece being in a legal position? A position involves placement of all pieces, as well as turn to move, castling rights, and en-passent capturabililty.

"legal chess position" is probably defined in terms of being able to arrive at it by legal moves from legal positions.