The Curious Case of the Illegal Double Check

Figure 1: The double check.
Figure 2: An illegal double check.
Figure 3: Stockfish 15 accepting and processing the illegal position shown in Fig. 2.
Figure 4: Two long-range pieces create a legal double check.
Figure 5: Two different long-range pieces in an illegal double check.
Figure 6: An illegal double check.
Figure 7: A legal double check.
Figure 8: A legal double check.
Figure 9: A legal double check.
Figure 10: The hypothetical position just prior to Fig. 9.
Figure 11: Illegal double check by Black with both kings illegally in check.

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