So the scenario is following:

- become root, mount FAT32 partition
- cd to that partition

I was playing a bit with the p7zip archiver, which is multithreaded and due to
some command-line options ran out of memory, but with a very strange message
(invalid argument to system call).

Then I tried to ktrace it to see which system call is at fault, and the machine
froze on me. Crashed HARD. Today when I came to work I saw the following thing
on the console:

uvm_fault(0xd72a3c0c, 0, 0, 1) -> 0xe

As for kernel DDB, I have these options in the kernel:
options         DDB             # in-kernel debugger
options         DDB_ONPANIC=1   # see also sysctl(8): `ddb.onpanic'

DDB didn't start. I could do just hard reset.