When I switched to spectator mode while riding a minecart, I should have either been kicked out of the minecart, spectating the minecart, or riding the minecart without controlling it.
When I did switch to spectator mode while riding a minecart, I was still able to control it's speed by pressing the "W" Or "S" Keys (Walk forward, walk backward keys)
To cause this bug:
1. Place a block.
2. Put some railing on that block.
3. Put a minecart on the rails.
4. Ride the minecart.
5. do /gamemode 3. You will still be controlling the minecart
Confirmed.