I still have a btrfs with a big problem: more disk space used than expected. The explanation was helpfully provided by btdu:

    Despite not being directly used, these blocks are kept (and cannot be reused) because another part of the extent they belong to is actually used by files.
    
    This can happen if a large file is written in one go, and then later one block is overwritten - btrfs may keep the old extent which still contains the old copy of the overwritten block.

With a clever script doing writes, deletes repeatedly, you can likely bring down any system with btrfs and it will bypass quotas.