I was trying to mount a samba share from ArchLinux, this share is unsecured, with a username of guest.

I tried a range of options

mount.cifs -o guest //ip/hdd /mnt/hdd
mount.cifs -o username=guest //ip/hdd /mnt/hdd
mount.cifs -o username=guest,password= /ip/hdd /mnt/hdd

Plus a bunch of other iterations and I kept getting

mount error(13): Permission denied'

After a bunch of searching I found a post that said you sometimes need to include sec=ntlm, and with this I got it to work

mount.cifs -o username=guest,sec=ntlm //ip/hdd /mnt/hdd