Minecraft wiki entry for Damage, associated text: Sand, gravel or concrete powder falling into the space the entity occupies.
Riding a pig, boat, or minecart into a one-block-high space.
Riding a horse, llama, or strider into a two-block-high space.
Standing where a tree just grew from a sapling, or where a huge mushroom just grew from a mushroom.
Standing where an end gateway appeared after killing the dragon.
Having a solid block pushed into the mob's head with a piston.
Sleeping in a bed surrounded by blocks or having a solid block above it.‌[Bedrock Edition only]
Being teleported into a block or having one placed onto the entity via commands (including /setblock or /fill).
When playing on a distant server, sometimes broken blocks can reappear due to lag, and if the player moves where the block respawned, it can provoke suffocation (for example, chopping down a tree by moving right below the trunk).
When water and lava meet, and create a cobblestone, stone, or obsidian block on the entity's head.
Standing a certain distance outside of the world border, configurable with /worldborder damage buffer (default is 5 blocks).
Summoning an entity inside a block through spawn eggs, commands, or with a golem or wither structure lying down.
Standing in 2 blocks of deep water in snowy and cold biomes where the topmost layer of water freezes into ice.