The bug
When you have a repeating command block (command: kill @e[type=falling_block]
) and you let an anvil (sand/gravel/dragon egg) fall on the ground it doesn't fall, only the texture disappears. So it is still there as a ghost block (you can't walk through). When you update the anvil by placing a block where the anvil was before it appears, floating in the air.
Code analysis
Code analysis by @unknown can be found in this comment.
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Is this still a concern in the current Minecraft version? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases. If this has been done, we can reopen the issue.
Keep in mind that the "Resolved"-Status on this ticket just means "Answered", and that we are waiting for further information on whether this issue still exists or not. We will reopen it as soon as the requested information has been delivered.
Confirmed for 1.8.2 pre-4. The entity will disappear immediately, and you can't walk through where it was. The impassible area is where the entity was when it died, not where it would have landed. Reloading the world fixes the bug, and causes the block to appear. It will not update, and will remain floating.
Also, if you update the invisible block, it will appear for a moment, then disappear again.
This bug (or a very similar one) will also occur if the fallingsand is teleported away.
Confirmed in 16w15b. Any blocks placed in the block where the invisible sand block is will disappear from your inventory and will make the sand appear. They will reappear in your inventory when you reload the world.

Please link to this comment in the description
The following is based on a decompiled version of Minecraft 1.9 using MCP 9.24 beta.
The method net.minecraft.block.BlockFalling.checkFallable(World, BlockPos)
only spawns the FallingSand entity but does not replace the block (this was maybe done to prevent FallingSand duplication?). The method net.minecraft.entity.item.EntityFallingBlock.onUpdate()
then removes the block if the FallingSand entity has a Time
of 0. It looks like the problem is that the server does not call the method before the FallingSand entity is killed. However as the client receives the packets delayed, it creates a FallingSand entity but runs the onUpdate()
method before receiving the packet to kill the FallingSand entity. This way the server did not remove the block, however the client did.
Edit: Expected behavior described in MC-72248.

Relates to MC-72248
Can confirm for MC 1.12.1.
Confirmed for 1.13.1.
Guys I know how to fix it. Place buggy blocks again (just place anything they will back) and use /fill for this cuboid of bug blocks. Helped for me ✔
Can confirm in 20w48a.
Can confirm in 21w03a.
Can confirm in 21w05b.
Can confirm in 21w06a.
Can confirm in 21w07a. Video attached.

I cannot reproduce the texture disappearance in 1.17. The block texture doesn't disappear in Avoma's video either. Have I misunderstood this issue?

@Marcono1234 Seems this is related to: MC-151488

This must have been fixed at some point. It happens in 1.18 but doesn't in 1.19, so somewhere in between.
I'm on 23w04a and cannot reproduce, the blocks are killed and disappear.

This has been fixed along with MC-72248.
Confirmed for 14w21b
Also happens with all FallingSand entities like sand, red sand, gravel and dragon egg