En Passant is a jerk.
Apparently, sometime around the 15th century A.D., some chess connoisseurs decided that the new rule allowing pawns to move forward two spaces on their first lurch was grossly unfair, and evened it out by introducing: en passant.
Now, there are two things about En Passant that are out of place in the game of chess.
First, the obvious: you take a piece in a square you never visit. It’s the only move in which the attacker does not replace the attacked. But that’s not so bad.
The bigger issue is this: En Passant is temporally dependent. It is the only legal chess move that requires knowledge of the previous board state. Given only the current state of a chess board, with no knowledge of how it got there, you can determine the validity of any move except an En Passant.
This is a pain when implementing three-person chess.