Sorry for the pun, but every ones favourite scarecrow appears to be bugged on the league! It appears that if Lee Sin uses his safeguard onto a person, and Fiddlestick's Darkwinds them, they are both insta-killed. The bug appeared of recent since the new patch and has been witnessed during many streams and games. This was captured by LoL replay by a Janna who was insta killed by the well know jungler Stonewall.
This video has been claimed as a fake due to LoL Replay often bugging out the HP bars of people. However, as this is from Janna's perspective, her HP bars are in fact up to date and we can clearly see that it is indeed a bug.
So what do? Well you may want to do re-create this with friends in a custom game but don't do it in a real one. It is after all an exploit and therefor you could be banned for doing it. Don't worry though, as riot should fix it as quickly as they fix their server problems... May god have mercy on us all.