Most filesystems don't allow creating hardlinks to a directory. However, soon after installing uutils (as part of an OS upgrade), I tried to create a symbolic link to a directory and forgot the -s.

uutils ln gave me the following error message:

ln: failed to create hard link 'target' => 'link': Permission denied

which left me unsure of what was going on, and I didn't immediately realise what I did wrong.

Meanwhile, in the same situation, GNU ln gave this more useful message:

/usr/bin/gnuln: target: hard link not allowed for directory

which immediately let me realise that I was trying to create the wrong sort of link.

This isn't technically an incompatibility, in that both ln implementations error on the same impossible request: but the more descriptive error message would be more useful for users who made the same mistake that I did.

ln version:

ln (uutils coreutils) 0.2.2