Expanding flakes (ones that will move when you pull outward on them) are not good placements for active gear. As the cam pushes outward on a fall the flake might expand and the the cam might pull out because it is past it's usable range. Hammering pitons in a flake like this will do it too. As you hammer a piton higher up the flake it loosens the lower piton because it expands the flake from above to the point that the lower piton might fall out.