I've got a panic when I tried to reboot the machine after running
some stress tests.  I'm not sure if this one is easily reproducible, but
here's the last message anyway:

boot() called on cpu#1
Switching to cpu #0 for shutdown
Waiting (max 60 seconds) for system thread vnlru to stop...stopped
Waiting (max 60 seconds) for system thread bufdaemon to stop...stopped
Waiting (max 60 seconds) for system thread bufdaemon_hw to stop...stopped
Waiting (max 60 seconds) for system thread syncer to stop...stopped

syncing disks... 6 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 
done
panic: assertion: node->lock.refs == 0 && node->ondisk == NULL in hammer_flush_buffer_nodes
mp_lock = 00000000; cpuid = 0

The kernel dump is on my leaf account as ~y0netan1/crash/{kernel,vmcore}.8 .