A colour flipped position, the board mirrored top to bottom with the colours swapped and the other side to move, must get exactly the same eval, since the eval is from the side to move's point of view. If it does not, some term is treating white and black differently, and that is the kind of bug nothing else notices. Eight workers each play random games, and at every ply they ask for the eval of the position and of its flip. Left to right mirroring is not tested because the piece square tables are not symmetric that way. Any position that fails is listed.