Yeah, I've had it happen to me a couple of times on a slant, once on a 273, and many times on a poly 318. All engines were super crusty inside and hadn't had an adjustment in decades by the look of it. After my second one wrung off, I decided I would just pull the rocker assy's off and clean them thoroughly if I found them that nasty again. Loose ones are much more common... especially if someone has removed all the adjusters and cleaned them on a wire wheel or ran a tap through the adjuster.
