Altera discovered that metastability on the address registers in any of the memory blocks can corrupt the memory contents. The following note was added to
Figures 7-9 through 7-13 in the Cyclone Handbook and will be added to Figures 8-13 through 8-20 in Cyclone II handbook:
Note: Violating the setup or hold time on the address registers could corrupt the memory contents. This applies to both read and write operations.
Also, on page 7-4 in Cyclone Handbook the following text was added and on page 2-28 of Cyclone II handbook, just above the footnote, the following text will be
"Violating the setup or hold time on the memory block address registers could corrupt memory contents. This applies to both read and write operations."
This will be fixed in a future version of the Cyclone II device handbooks. This was already fixed in Cyclone Handbook version 1.5.