I spent so long looking for it, and I've finally gotten it! THE DUPLICATION GLITCH!!!!!!
You can execute it by getting a permablock in a wall, pushing an hourglass into it, undo, redo by pushing again, and reset level. Then you can push that same hourglass into another block and it won't release the old block. As far as I can tell you can repeat this infinitely. This screenshot was not taken with the level editor.
I'm pretty sure this works with any non-hourglass block that's stuck in another non-hourglass block with lower permanent priority. This includes non-pushables, like flags or Xs. I haven't been able to get it with a wall because it seems like every other block has higher priority when permatizing.