I had a similar debacle, when I managed to corrupt a btrfs file system to point it wouldn't mount again...
I was preparing it to have as my main system on bare hardware. I had accidentally mounted the same block device simultaneously in the host and guest: kablamo silent corruption and all 5 hours of progress lost.* :(
*shred the guest VM, host was ok.
I touched a piece code that was +10 years old, according to git. Should I be scared? Will my change it survive??