On a night out in London I lost my keys, not realising they'd dropped or been dipped out of my jacket pocket. I got back to my car to find I couldn't find them.
The really bad thing was that I kept my house keys and car keys on the same key ring so they'd all gone. With no-one having spares to my car or my flat I not only had to pay the garage to provide me new keys but also a locksmith to get me back into my flat the next morning.