When I take a business trip, I rent a car from an Avis place that is three-tenths of a mile from the parking lot where I work. I leave my own car in the company lot, and walk to the Avis to get the car. I take the rental car key-ring and attach it to my regular keyring, so that I have one ring, ensuring that no key will be lost. Please bear these things in mind.
When I returned the car to Avis, I took my keys off its ring and handed the car keys to the Avis rental agent. "What's the car mileage?" she asked. I had not thought to check, because usually the rental company uses an electronic thingy to get the mileage.
"I'll go check," I said.
"You'll need these." She handed back the key ring, and I saw, with horror, that the key for my own car was still attached. I might not have been able to drive my own car anywhere. I extracted my own key, gave her the mileage, and walked to work.
I did about two hours of work and then walked to my car. I tried to open the locked door with:
Oh hell! With the Avis car keys, which were still in my pocket. I drove at once to Avis, wondering why they had not called, and whether they would charge me an extra day for keeping the keys.
I walked into the Avis place. The same agent was on the phone. I quietly handed the keys to her. Her eyes widened, we nodded to each other, and I left. Phhewwwww.