How to reproduce:
1. Join a server with allow-flight=false in its server.properties so it will kick out players in survival mode or adventure mode that are floating for longer than 5 secs.
2. Make sure the player is in survival mode or adventure mode.
3. Jump
4. When the player is in the air, immediately ride a horse/llama or minecart (Boat does not work)
5. Keep riding for at least 5 secs and you will be kicked out of the server because it thinks you keep floating for 5 secs which the player might be using a flying plugin.
Possible causes:
The floating timer is not reset until you reach the ground or some fluid. If you ride something other than a boat when falling you will not touch the ground.