Falling sand, gravel and concrete powder

The build pastes fine, and then sand, gravel, concrete powder, anvils or dragon eggs start raining down. Sometimes it happens immediately, sometimes only when a player walks near and the chunk gets updated. Beaches slump, desert builds lose their walls, and anything decorative made of sand ends up in a pile.

These blocks fall whenever there is air beneath them and something tells them to check. Pasting places them in an order the game did not choose, so a block can exist for a moment before its support does — and that moment is enough. It is not a fault in the file: the same build sitting in the original world is perfectly stable, because it was never asked to re-check.

1. 1Paste without block updates. In WorldEdit this is //fast or the no-update paste flag, depending on your version; FastAsyncWorldEdit does it by default. This is the real fix and it prevents the problem instead of repairing it.
2. 2If it has already collapsed, //undo, then paste again with updates off. Undo is reliable here — do not start rebuilding by hand.
3. 3For a build that genuinely has floating sand by design, replace the hidden support with a solid block, or swap the sand for its non-falling equivalent where the look allows it.