There are a few issues with the latest version of Keynote running on Mountain Lion.
The following routine resets the installation of Keynote and sorts out many of the issues users are having at the moment.
Uninstall all of iWork if you have this, otherwise uninstall Keynote if other iWork applications are not installed.
Use the free application Appcleaner to do this as it will remove the applications properly. It can be downloaded here: Appcleaner Download
empty the trash
Reinstall Keynote or iWork using the original method on purchase using the installer file or installed from the App Store
shutdown the Mac and restart holding down the shift key until the apple logo appears
let the Mac complete the start up procedure completely, it will take longer to start as it is repairing software on the hard drive
go to Applications > Utilities > Disc Utility > First Aid > Repair Disc Permissions
when this is completed restart the Mac normally
Let us know how you get on
I did steps 1 to 3. However, when I restart the Mac holding down the shift key the Mac does not complete the strat up procedure in any reasonable amount of time.
I reinstalled Mountain Lion, removed iWork with appcleaner, reinstalled iWork 9.0 from CDs, updated iWork to the latest version (iWork 9.3), and the problem is still there: when I start keynote and I try to save the file I get "An unexpected error has occurred. Please quit and reopen Keynote."