UPDATE – Sept. 19 at 8:30PM ET: Infinity Ward has stated that they have deployed a fix to address the issue that allowed users to switch input devices during a game in Modern Warfare.
A bug in Call of Duty: Modern Warfare Cross Play functionality is letting users switch input devices while in a lobby or in a match in the game.
This is allowing some users to switch their input device mid-game from keyboard mouse to controller or back to either input device.
TeeP showcased this in a video:
This was an issue that many people did fear about with cross play in Call of Duty: Modern Warfare – the ease of which users can change input devices after connecting to a game or set in a lobby.
Infinity Ward has not addressed this as of now, but it does seem to be a big concern area for users who were not wanting to experience this. Players can fully opt out of cross play in Modern Warfare and avoid playing with users on other platforms.