This has now been seen several times on two different clients (so it
probably isn't a hardware issue such as bad RAM; the second client has
ECC). The backtraces vary slightly but always involve NFS. I haven't found
a way to recover without a reboot.
I'm now upgrading these systems to the kernel from jessie-backports; we'll
see if the problem still occurs with the newer kernel.
I'm wondering if this might be a regression in 3.16.36 since the problem
appeared recently. I haven't seen it on Ubuntu's 3.13 kernels (of which
we have many instances in production) either.
Bisecting is difficult since I still don't have a sure-fire trigger for the bug.