As stated in the title, Falling Block Entities (sand, gravel, concrete powder, ect) can become desynced between the server and client.

When this happens, the visual location of the falling block displayed to the player becomes irrelevant, as the falling block will snap to, and turn back into a block when it lands server side. This can:

• Cause the falling block to rest on top of another, remaining as a entity if the falling block lands client-side before it lands server-side.

• Cause the falling block disappear out of the air and snap to a new position if the falling block lands server-side before it lands client-side.

This usually occurs when using pistons/slime blocks to move falling block entities, if proper timing is used. This can make technical builds involving the movement of block entities confusing especially when specific results are required, as the falling block may appear to behave a specific way, but then suddenly snap somewhere unintended or unwanted.

Attached is a image showing the redstone contraption that seems to reliably replicate this issue, showing it layer by layer, with a checkered grid for convenience. Also provided are three links to videos showing the bug with the contraption, one from earlier on which this issue was first discovered, another using the contraption shown in the image, and a third recorded a bit after showing the other scenario of this bug occurring. (The contraption is altered a bit with different timings.)

Note: Occasionally the falling block will be synced properly when pushed as shown in the video, however this seems to be uncommon, and also seems to be influenced by position in the world.

fb_bug_00.mp4
fb_bug_01.mp4
fb_bug_002.mp4