I've told this before, but DR JRand74 mentioned his, so...
The One That Got Away
I bought my 1st printing of
Atlas Shrugged in great shape with a wholly intact dust jacket for $1 at a Goodwill while on a visit to Miami in 1980. I wasn't into collecting books at all then, but I knew even then that it was a prized volume. I also knew very little then about protecting valuable books, but I was a careful person and it was with me in good shape for a couple of years ... when one of my best friends at the time asked if he could borrow it. And then I moved east, a tumultuous time when we all literally lost track of each other and I had plenty of other things on my mind.
Folks, never loan a valuable book.

Years later I found another copy of it, also in great shape, 9th or 11th printing, I forget (it's in a box where I can't get at it to check right now), and I keep it just as a reminder of that lesson and as an angry fist shaken in the general direction of the universe.