I had to shells open when adding users and the server stopped while adding user # 498 (I added 2 users before by hand).
The server was not able to write to disk, all processes wanting to write were in D state.
I couldn't connect with ssh but httpd answered one request.
The second shell was operational, I could do ps and df, but it stopped suddenly and I had to power off.
After power on I synced accounts and restarted adding users. It kept adding users until I interrupted the process, after at least 10 users more.
I suppose it's a subtle bug.
I'll try to look into it.