The bug

You are visually able to place a block on sand that is just starting to fall because the client thinks it is still a sand block even though the server knows it is an entity. Sometimes you are actually able to place the block, but it is very difficult to reproduce. (Has same reproduction steps; if you need me to split it into two issues [since one is server-side and one is client-side] let me know )

Steps to reproduce

1. Place sand somewhere where it floats
2. Immediately try to place another sand block off the floating sand block
3. Notice it appears to let you for a split second