[LegionMammal978]

One thing I was surprised to learn a couple years ago is that users and groups aren't really tracked much by the Linux kernel: they're just numeric IDs that track process and file ownership. So if you setuid() to a user ID that doesn't exist in /etc/passwd or anywhere else, the kernel won't stop you.

[latchkey]

If I have a file on machineA with uid10001 and I copy the file to machineB, I might want it to retain that uid, but it shouldn't matter to machineB that it doesn't map to a real user.

[rafram]

Hopefully that user actually doesn’t exist on the second machine!

[MrDarcy]

You’ll see this observation all the time building containers.

[danielrico]

Don't if you only run them with root user.

[kurisufag]

or with ipa-esque authentication schemes and shared mounts

[dimatura]

And NFS!