In my case, "Error 15" occurred right after Fedora installation. It seems that I didn't check somewhere during installation to write GRUB to the /dev/sda disk. After first reboot (while finishing Fedora installation), GRUB displayed "Error 15" message and booting was stopped.
The solution was to boot PC to "Rescue mode" and install GRUB into the boot sector (MBR). Recent Fedora CD/DVD installations have rescue option so it's only needed to start PC from CD/DVD. On initial menu, choose "Rescue mode" and wait until terminal will be ready. Now it's only needed to execute the following two commands:
# chroot /mnt/sysimage # grub2-install /dev/sda
chroot will start interactive shell with defined root directory (/mnt/sysimage is mounted hard disk), while grub2-install will install GRUB to the MBR of first hard disk. If your configuration contains several hard disks, you should check where to install GRUB (it's not always needed to install GRUB to the first disk). Anyway, after these commands are executed, reboot PC and the problem should be solved.
This post is short, but it can be very handy if PC only prints "Error 15" message.