Strange dungeon generation

While exploring in a mineshaft, I came upon what almost looked like a hole in a cobblestone and mossy cobblestone forming the shape of a likeness to a nether portal, but with one less block in proportions in both axis. When I mined around it, it was just a single wall, with the hole, off to the left of center, and it was inside the mineshaft. The image only shows it before I mined around it. I am unable to send the world, as some kind of major mess from removing an addon somehow caused first chunk errors, then entire chunks, and parts of chunks to disappear, both even affecting already loaded chunks.