The best move in the following FEN is to long-castle, but it's illegal according to current chess rules. This is super serious bug, it's urgent to be fixed.
r2k1r2/ppp4Q/2n4b/3B1bq1/5R2/3P4/PPPB2PP/4R1K1 b - - 0 1
r2k1r2/ppp4Q/2n4b/3B1bq1/5R2/3P4/PPPB2PP/4R1K1 b - - 0 1