ah, the panic that comes from unwittingly mounting overtop of an important system directory like /usr/local caused by using `zfs create -p $POOL/usr/local/subdir`

It creates a /usr/local dataset in the process (as instructed by the -p) and then mounts it atop the existing /usr/local with my local packages.

That was 10–15min of semi-panicked frustration (I could tell from `zpool history` that data should still be there; I just had to figure out why it had vanished and recover sanity).

Here's hoping it saves someone else headaches if they `zfs create -p` something and suddenly the previous contents of things get masked.