I was creating a ZFS mirrored pool on a FreeBSD cloud instance with two identical data disks, like a gentleman:

$ ovm create disk test1 in SY3 with 100g on tier SSD
$ ovm create disk test2 in SY3 with 10gb on tier SSD
$ ovm attach disk test1 to VM test
$ ovm attach disk test2 to VM test

And then on the VM:

# glabel test1 /dev/ada1
# glabel test2 /dev/ada2
# zpool create [options] mirror tank \
    /dev/label/test1 /dev/label/test2

Output:

==> invalid vdev specification
==> use '-f' to override the following errors:
==>   mirror contains devices of different sizes

Whoops! Turns out they were less identical than I realised. You’ve have thought the monospace font not lining up during provisioning would have been a clue. Either way, good to know. Also, derp.