Unfortunately, yes. I had a pistol slip out of a "generic" horizontal shoulder holster. I thought the thumb strap would hold the pistol in even though the holster was designed for larger pistols. (learned a good lesson there) Somehow, after I had completed about a mile of my daily walk I felt the pistol slide down inside my jacket. Before I could catch it it had hit the ground and skidded a few feet ahead of me on the asphalt hiking path. There were joggers coming in the opposite direction so I quickly picked up the pistol and walked off to the side so I could safely reholster. I don't know if they knew what had skidded along the path, but I was totally embarrassed anyway. Figured out my exertions had caused the thumbsnap to slip over the beavertail and allow the pistol to fall out. Got a small abrasion on the front corner of the slide...damn!
Had another pistol almost slip out of a nylon OWB holster. I sat down on a chair with a pillow at the back. When I stood up I felt the shifting pistol and quickly held it against my butt. (Luckily I was wearing a jacket) I calmly walked to the bathroom and reholstered. Hopefully they just thought I had a butt-pain. Never used that holster again.
NOW I use holsters that fit pistols snuggly. Haven't had a issue for years now and I carry much more now, and much more securely.
edison