Especially sticky pistons that move too quickly and leave the slime block behind is capable of producing fake blocks that exist, prevents you from moving, that you can stand on top of without falling but does not yield any items when destroyed in creative mode.

To recreate the problem:

Step 1:
If you have 2 sticky pistons facing towards each other, with 2 blocks in between them. The one block must be air and one must be a slime block.

Allowing the slime block to be shuffled back and forth while the pistons remain at the same position. To keep them there I used obsidian on the opposite ends as pistons won't push them.

Step 2:
into the one piston have a repeater (1 tick delay, default placement setting, don't change delay) into the other piston a solid redstone dust wire.

Connect the repeater to the wire with another 4 pieces of redstone dust so that especially you have a very short U with wire on the one side and a repeater on the other.

Step 3:

Place a comparator facing into any of the 4 pieces of redstone dust you just placed down and change it to subtraction mode. Then add 1 more piece of dust next to it connecting the wire to its side to allow it to repeat a pulse.

Final Step:

This contraption on its own won't do anything, but the moment you add a strip of slime blocks parallel to the line of sticky pistons you made in step 1:

Especially it was meant to move the line of slime blocks either up or down to create an elevator and it does, however it leaves behind a trail of fake blocks in both directions.

Logging out and back in removes them. When other blocks get pushed through the fake block it also gets erased. However its still a visual and physical bug.