Your computer may start up from the wrong System Folder when multiple versions of Mac OS X are installed.
Symptom
The computer does not start up from the System Folder selected in the Startup Disk pane of the System Preferences application.
Solution
If multiple version of Mac OS X are installed on an affected computer, the Startup Disk setting may be ignored.
On the affected computers, you should not install different versions of Mac OS X on different partitions or disks.