I remember reading about Patrick Combs [1] depositing a "sample check" from some junk mail and it ended up clearing, despite it saying "not negotiable" on it. I wish his original story was still on the web because it included a ton of detail about what happened, but alas ...

Previous mentions of Patrick Combs on Hacker News: https://news.ycombinator.com/item?id=4344720 https://news.ycombinator.com/item?id=2020631

[1] The best link I could find today: https://www.reddit.com/r/todayilearned/comments/37b74g/til_t...

EDIT: added additional Ycombinator links.