Pre-2020, but I had a BTRFS filesystem with over 40% free space start failing on all writes, including deletes, with a "no space left on device error". Took the main storage array for our company offline for over a day while we struggled to figure out wtf was going on. Supposedly this is better now, but basically BTRFS marks blocks as data or metadata, and once marked a block will be reassigned (without a rebalance). Supposedly this is better now, but this was after it had been stable for a few years. After that and some smaller foot guns, I'll never willingly run BTRFS on a critical system.