I nearly lost all pictures I had saved of my waifu because I was using an external HD at the time that I used on both Mac and Windows. I had a temp folder on my Mac that I saved into whenever I was traveling. It piled up, so I figured it was time to merge the folders by dragging the folder from my Mac to the external HD (they had the same folder name).

The only problem was.... Mac does not merge folders. It overwrote it. I cried, freaked out, asked every tech friend I could for help, and I nearly lost everything, but I made sure to not shut the drive off. A few hours later, I plugged it into Windows, and Recuva to the rescue!

So while it was a happy ending, that was a HUGE scare for me.