It wasn't good experience on upgrading Mac OS X to Mountain Lion comparing to upgrading to Lion. After downloading the operating system, double clicking the installer, 'Install OS X Mountain Lion', and waiting for restarting the system, the error message occurred. Upgrading failed. I also tried the clean installation. It also failed. 'There was a problem installing Mac OS X. Try reinstalling.' was the last error message. The final solution that works for me is resetting my Mac's PRAM and NVRAM.