Thanks for all the suggestions. I have been withdrawing the dark slide completely since I started learning. Mostly because I'm fairly certain I would not do it correctly otherwise, ha, ha. I don't think I'm inadvertently sliding the sheet too far down towards the flap end when loading because I tried that a bit ago to make sure, with some scrap sheets I keep around. If you do, you can't close the flap, nor fully insert the slide and lock it without it being obvious. Now, on occasion, I have 'forgotten' to remove the dark slide, rather embarrassing. I re-cock the shutter and hope any uninitiated nearby think it was a deliberate practice shot.
Right now, I seem to be focused (no pun) on Dann's thought about not inserting the holder all the way. I thought I was, but perhaps not. I will load a couple of holders, even one that I know this happened in, and see what happens. Thanks all for making me think.