I'm getting this warning from my server console (and just happens to be for the same player each time). I'm the only mod of the server so I know no one is using commands to teleport themselves and even if they did it would leave a log. What causes this warning?
This warning is generated every time a player moved to a position or at a speed which the server thinks is impossible. This can happen spuriously, as the assumptions the server makes aren't quite the same as what you can actually do (the server doesn't analyze the player's exact inputs), or it can happen due to an attempted cheat, or due to the world changing as the player attempts to pass through it (e.g. player moves through a block the server knows about but the client hasn't seen yet due to latency).
Whenever this happens the server sends a message to the client resetting the player’s position to the last valid one (this can be noticed from that player’s perspective as a sudden shift).