Gertie:
I think almost everybody has left the master switch on at some time.
I was very frustrated in one of the Paris peripheral airports a few years ago. I had refuelled in a hurry, then walked a hundred yards to the smart terminal to meet my buddy, who was filing our flight plan and getting some coffee. When we were ready to go, we went through security with the passengers of two low-cost airline flights and waited at the gate to be let out onto the apron. I looked idely across at 'AA, a hundred yards away, sitting in the gloom.
I was horrified to see the beacon flashing. It must have been nearly an hour since I'd taxied to fuel. What else had I left on? There is no avionics master, so in addition to the intercom, gauge circuits, magnetos and turn-coordinator (which come on automatically with the master), I could easily have left on the taxi light or some avionics!
I told the security guard that we needed to go to the aircraft immediately and pointed out the flashing beacon. His reply was we could not move onto the apron until the two airliners had departed. One of them had not even landed yet.
Eventually, we pursuaded him to take us out, half an hour later, after the inbound had landed, but before the two jets were ready to go. It seemed more like two hours at the time, and the little flashing light seemed to be getting dimmer and dimmer.
The engine definitely turned over much more slowly on starting. It felt like it started on the battery's last gasp.
Were we glad to be in the air!
It was only later, after we had landed, that I wondered if we should have run the engine for 20 minutes or so on the ground, before departure, until the charge rate slowed down. I've heard one or two horror stories of what happens when batteries are charged too fast!
I've never left the master switch on again (so far)..............................
.................but I have left the magneto switch on!!
Alan.