If you stand on a boat (not riding it) when it is partly over the edge of a drop of 1 block or more, and you are positioned over the drop, you will be kicked for flying.

Steps to reproduce:
1. allow-flight=false (in server.properties)
2. Gamemode = Survival
3. Place boat partly over a drop
4. Stand on the boat, while at the same time being over the drop
5. Wait 5 seconds