Update OS X Mountain Lion to version 10.8.3 or later to address this issue.
- From the Apple () menu, choose Software Update. The Mac App Store will open.
- Click Updates.
- Choose and install the OS X Mountain Lion v10.8.3 update. Alternatively, you can download and install the update manually here.
- Once your computer restarts, open Boot Camp Assistant and follow the onscreen instructions to install Windows 8.