I had an unusual problem I’ve never experienced using Nikon cameras and lenses since the late 1960’s. I recently literally could not unscrew the Nikon rear lens cap from my Nikkor 28mm Ais f 2.8 lens! I even tried a large pliers. I finally removed it by carefully sawing it nearly in half with a hacksaw (fine tooth metal saw). See photos. It appears the lens cap I had used was a newer one which has a much wider outer rim than older lens caps. The width of the rim of the cap became jammed into the indented outer rim of the lens. See the photo of the two types of Nikon rear lens caps. I was able to do this without damaging the lens, thank goodness.