I think the lesson there is that "the spec is what the program currently does" doesn't work well for the behaviour around rare errors. (POSIX was a red herring

I think the lesson there is that "the spec is what the program currently does" doesn't work well for the behaviour around rare errors. (POSIX was a red herring: Linux has never limited itself to POSIX, and anyway POSIX didn't promise that doing things the way the ext4 maintainer recommended would work either.)