In if statement here we should check _backupPlanIdx, not _bestPlanIdx https://github.com/mongodb/mongo/blob/1d93d09948c29534f7c82950ba7771ac0f4da22c/src/mongo/db/exec/multi_plan.cpp#L367

It is relatively harmless, but causes debug mode to show errors, because of an old C++ standard issue: https://stackoverflow.com/questions/22915325/avoiding-self-assignment-in-stdshuffle