Minecraft wiki entry for Block, associated text: When the block is removed by a piston.[1]
If the block is affected by gravity and falls into an invalid space (If the block is gravity based and drops from a torch, for example).‌[Java Edition only][2]
Anvils that are destroyed by running out of durability or falling. Only particles are missing.[3]
If the block can be washed away and is washed away by a flowing fluid.[4]
If the block can be replaced by other blocks and is replaced.[5]
If the block is one of a few blocks such as rails or redstone wire, supported by another block and its supporting block is removed.[6]
If the block is leaves and decays.[7]