I just wasted 10 hours piecing back together some assets that I accidentally deleted. These assets were already mostly finished and I just had to send them to my client who had already paid for them. I tried to use various tools to recover the lost files but most of them were already corrupt. Luckily UE4 autosaved a decent amount. I only had to spend a few hours piecing everything back together and redoing some blueprints and materials.
Ironically I usually work in Dropbox but I didn't for this project because that hard drive was almost full.
It was still quite a costly mistake as I wasted 10 hours but I could have easily lost everything and it would have cost me several days to rebuild it all.
I wouldn't wish this upon anyone so please make sure you have a functional backup system at all times. Hard drives can fail or you can make stupid mistakes like I did. (I won't get into how it happened exactly but Windows messed up something and I have a bad habit of using shift+delete.)