Why not to cut into the meat when scoring duck breasts?
The little bit (about 4 - 5 grams after cooking) of fat in the breast itself will render out faster, and you could end up with a dryer over-crispy duck breast by the time it's done in the center. A tiny ding shouldn't present a problem, but if you actually get the whole or most of the edge of the blade into the breast, you'll have to pay close attention to heat as you cook it. If you score it that deep, you're probably better off poaching it in butter after searing.

The juices in the breast make it moist, and you definitely don't want to lose those either, but that tiny bit of fat in the breast once you've rendered out what's in the skin makes it delicious. You really want to keep that in.