I had a free book reward from Thriftbooks, which I used for a copy of Long Dark Teatime of the Soul by Douglas Adams. When it came, I was kinda bummed that some kid had scribbled on the title page, but eh, it was free. At least it was a first edition in otherwise good condition, I won't complain.
As I was reading, I got a little itch in the back of my mind, and it of curiosity looked up his signature. Turns out, it looks like some kid's scribbling
Probably why no one realized.
So I got a signed first edition, which goes for about $200, totally free.