You are probably aware of the existance of chkbfs. This tool checks the file system for errors, and corrects them, if possible. Nothing is perfect, so you might not even be asking yourself why a journaling file system comes with such a tool.
This is a companion discussion topic for the original entry at https://www.haiku-os.org/blog/axeld/2007-10-05_why_bfs_needs_chkbfs/