In the Quartus® II software versions 4.2 and 5.0, designs using the M4K blocks in Stratix® II devices may be susceptible to intermittent read failures due to an issue with the configuration bit settings for the M4K memory blocks.
Refer to the Stratix® II FPGA Family Errata Sheet for a description of the issue and an explanation of the memory configurations that can be affected. This issue does not affect your design if compiled in the Quartus® II software versions 5.0 SP2, 5.1, or above.
To determine if your specific design is affected by the issue, refer to the following solution: How can I find out if my design is affected by the Stratix® II M4K issue if I'm using the Quartus® II software version 4.2 or 5.0?