"Falling blocks" refers to sand, red sand, and gravel.
Blocks that will prevent falling when directly underneath the placed falling block are: skulls, torches, slabs, switches, redstone, etc. – basically anything that doesn't have a valid top surface for falling blocks to land on
To reproduce, simply place a block without a valid top surface to catch the falling block when it's dropped from a higher point, then place a falling block directly on top. Rather than falling, the block will simply sit where it was placed
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