I've been known to make mistakes when I print. I load the paper in the wrong direction or backwards. I use scrap paper from old jobs to run tests. There are a lot more possibilities but I won't bore you with the details. Most of the time these accidents just end up in the recycle bin but once in a while the combination of ink, paper and design become more than just a mistake. These oddities occur often enough that I have started a collection. It's a pile of happy accidents that serves as inspiration for all kinds of things.
Once, while I was printing a script, I did a test without ink. After putting ink on the machine, I hit the card again. It was offset by the exact width of the letter stroke and green ink was laid down a little below and to the right of the blind run. The effect was just like a drop shadow. Into the happy accidents pile it went.
Then along came the Edwards & Son business card. This design was already using a drop shadow but I thought I might try printing a blind run instead of using the red ink. The effect is strikingly different. I love how it seems like there must be some ink there but it's just a play on the light.