Can escaping one check accidentally create a new illegal position?
These answers restate the rules implemented by our open-source engine, which follows the WXF laws and is regression-tested against the official 110-diagram repetition casebook. Where an answer refers to a specific position, that position lives on the linked reference page, where the build verifies it against the engine before publishing.
Direct answer
Yes. When a general moves to get out of check, each candidate escape point is tested against every rule, not just the immediate attacker. A point that would remove the check but put the general on a clear file facing the enemy general is not a legal escape at all, the flying general rule rules it out just as firmly as staying in check would.
Why this catches players out
It is easy to scan only for the piece giving check and assume any point outside its reach is safe. A general in a sparse endgame frequently has fewer real escapes than it looks, because one or two of the obvious squares are actually illegal under the flying general rule rather than merely unsafe.
What this means for calculating mate
When you are checking whether a position is checkmate, you have to test every square the general could physically reach against all the legality rules together, occupied by a friendly piece, attacked by something else, or facing the enemy general, not just against the piece delivering the current check.
Related questions
Does this only apply to the flying general rule?
It is the clearest example, but the same idea applies generally: any square that would leave the general in check from anything, including a different attacker, is equally illegal.
Can this ever turn a check into checkmate unexpectedly?
Yes, a position that looks like it has an escape can be mate once the flying general rule removes that one remaining square.
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