I'm using smbfs and vnd on a freshly booted 3.99.23 system. I copied  
the fs image to /tmp, mounted the vnd0a device, copied a file to it  
and a few seconds later I got this:

panic: buf mem pool index 23
Stopped in pid 16.1 (ioflush) at netbsd:cpu_Debugger+0x4: leave
db{0}>

I can't get a backtrace because usb keyboards don't work in ddb with  
generic, so off to build a new kernel without pckbdc/pckbd and with - 
g to try to reproduce.