Why does the Quartus II software overwrite my database compiled in a different version without giving a warning?

Due to a problem in the Quartus® II software version 10.0 and later for Windows, if you open a project that had been compiled in a different version of the Quartus II software, the software may overwrite the compiled database without asking you whether you want to overwrite the database or keep the original.  This problem can occur in the following cases:

• Enabling the option Re-open current project at startup on the Options dialog box
• Opening your project by pointing to the File menu and selecting your project from the Recent Projects sub-menu
• Specifying your project when starting the Quartus II software from the command line, for example:

  quartus <project>.qpf

To avoid this problem, when opening a project that had been compiled in a different version of the Quartus II software, avoid the above cases and select Open Project... from the File menu .

This problem is scheduled to be fixed in a future vesion of the Quartus II software.