I had a couple of wooden 4x5 holders that leaked. Found cracks on the side that let in the light. My only other problem was with some older film holders that did not match up correctly with the back of the camera, and fogged the film as soon as the dark slide was removed.
I now have tested all my holders, took the lensboard off then in the dark shone a light from the front. Found quite a few wooden Graphic holders that were slightly warped. But all my Fidelity, Lisco and Riteway holders were fine.