Here's another tip that paid off for me yesterday. Always carry a spare bar nut or two in addition to the ones on the saw. I was changing chains with the saw on the tailgate of my truck. After I removed both nuts, I flicked one of them with my finger and knocked it off the tailgate between the truck bed and the bottom edge. Somehow it found one of the two slot holes in the tailgate and dropped right in, making it impossible to retrieve. The chances of doing this again are almost nil, but it happened.
I had a spare bar nut in my tool box so I could finish the job, but running a large saw on one nut only is not a cakewalk and recommended by nobody. That nut may rest inside my tailgate forever because it will likely take me several hours to get it out and hardly worth the time and effort. BTW, any tips on that process would be appreciated.