On a multiplayer server, while placing boats, if you place a boat in a suitable open space, but while you are too close to the location, you will swing your hand like you are placing the boat, then the boat will disappear from your inventory, but no boat will be placed or dropped, and you cannot get it back until you disconnect and reconnect to the server.

In addition, the boat is still in your inventory the whole time, and other people can see you holding it, and you can even press q to drop the boat, but you cannot see the boat in any way in your own inventory until you reconnect to the server.