Aero Commander 500 pilot reported left engine failed during initial climb. Pilot landed safely and discovered post-flight left engine fuel shutoff switch was in the off position.

Narrative

On Day 2 I was conducting a personal flight with Person A; the aircraft owner. Both of us have sever hundred hours in type. The aircraft had recently had maintenance performed and I had flow the aircraft on Day 0 as a return to service flight (RTS not required). The aircraft flew perfectly with no defects noted.On this day Person A was the pilot flying. He performed all flows and checklist for before start; before takeoff; an engine run up and takeoff. As we taxied towards the hold short line I observed him turn on the boost pumps; pitot heat; and strobes. All seamed normal.Person A applied full power and we started our takeoff run. Near rotation I noticed the left engine fuel pressure was slightly low. I checked the fuel flow; manifold pressure; and rpm. All was normal. The fuel pressure being low on the left engine was a common problem and had been previously checked and there were no fuel pressure issues found. I think this created a 'bias' in my mind. Meaning I was conditioned to low fuel pressure being normal on the left engine. Shortly after rotation (50 feet AGL) I observed the left engine fuel flow starting to decrease. I quickly looked up and ensured the boost pump was on. The left engine failed at roughly 100 feet. I immediately assumed command of the aircraft and performed the engine failure after takeoff procedure and feathered the propeller. We landed without incident.I am not sure what happened. After landing I found the fuel shutoff in the off position on the left engine. I asked Person A if he shut off the fuel; he stated he didn't. This makes no sense to me. We don't normally operate the fuel shutoff switches. You can't start taxi of takeoff with these switches in the off position. The fuel valve closes and the engine shuts down very quickly; roughly three seconds. I really don't see a way the valve could have been selected off prior to the takeoff roll and make it to 50 or 100 feet. All switches appeared to be in the proper position prior to takeoff; but I can not explain why it was off after landing. I did not see Person A shut the switch off.