If you forget to switch back to the main tank when filling you will run out and not have a reserve. That’s one reason it’s a good idea to always turn off the gas, you will remember to turn it on the main when starting.
Many years ago I delivered the newspaper. The sunday comics were delivered to me on Saturday, I had to insert them in Sunday’s paper. Since Saturday’s paper was smaller I started delivering inserting the comics in the Saturday paper. The newspaper company got wind of this and told me to stop doing that. I never got a complaint about delivering the comics with the Saturday paper, but when I quit doing that and started delivering them on Sunday’s again I got several complaints.
Even alone, sometimes you don’t always get your way.