1-Get in a boat

2-Hold forward, and dismount (shift key) while still holding forward

Result: Your hands/the item you're holding drops down when you hold forward (presumably to indicate that you're using your hands to row), but when you dismount your hands are not restored to their intended place. You can break blocks in this strange state, but you cannot break the boat.

Exiting the game and reopening my world restores my hand/tool and everything works normally.

I'm astonished I can't find this elsewhere because it's so obvious, so apologies if it exists but I didn't find it.