Solved a starting mystery that might help someone else out. I just replaced the clutch in my 2009 Clubman S, and when reassembled the car would crank but not fire. It had spark and fuel, but I could not get the engine to run, even with starter fluid squirted into the intake. And no codes were thrown that suggested the source of the problem.

I finally noticed that the rear brake lights were constantly on when the key fob was inserted. It turns out of the gazillion connectors that I had unplugged during the clutch job, I had forgotten to re-connect the brake light switch, which was disconnected while replacing the clutch master. Plugged the connector in and the car fired right up. There must be some immobilizing feature with the switch disconnected.