(I know this bug is known and common, just wanted to make one detailed)
The world loads slowly when you join it, and chunks load slow and glitch out when you load them for the first time. Happens on servers as well. you cannot move while it happens initially, but on junk borders, you may fall off the edge for a while and get stuck until it loads.
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migrated
migrated
This should be a complete list of all things that dupe: Chunk Format Entities and Block Entities
UPdate info:
When the chunk does not load, it is possible to sneak over the edge and break blocks underneath the ground.