When connecting a sticky piston with blocks attached to a slime block to an observer rapid pulser (may work with other pulsers as well), the blocks attached to the slime block seem to duplicate, but breaking them in survival yields no blocks. Other than disappearing when broken or updated, the phantom blocks do act like real blocks. You can stand on it, walk on it, etc. However, a block update will cause it to disappear. Video showcases this bug and how to reproduce it. Skip to 0:52 if you want to skip the setup.
The video