When placing a boat on the corner of a block with water on top of it, but with no water on the blocks adjacent to it, the boat will fall through the floor.
It's hard to explain with words, so here's a video:
https://www.youtube.com/watch?v=-h25jhAxvZE
Linked issues
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violine1101
Retroactively closed as a duplicate of MC-122493, which has been fixed in a 1.13 snapshot.