In the past I have experienced issues with 4x5 film moving within the holder during long (~20 min.) exposures on damp nights (holder tapping or not, camera usually dead level). (typ. Fidelity/Lisco holders)
I've recently acquired a couple of Grafmatic holders and before heading out in the dark, I thought I'd ask if anybody has experienced film movement (shifting, buckling, etc.) with them during long time exposures?