Does this happen often?

Going through my bookshelves picking things to give to the library or sell, I found two autographed books I'd forgotten I had. I bought them at used book stores for a couple of dollars each in high school or college, and didn't notice until I got them home that they were autographed. Separate bookstores and a couple of years apart, too.

To be clear, these books are probably not actually worth anything - they're The Spirit Ring, by Bujold, and Hero's Song, by Edith Pattou, both pretty scuffed and made out to specific people.