memc0 at mainbus0.
This is the normal autoconfiguration message indicating that the Memory Controller Chip has been found and attached to the main processor bus.
memc0: Correctable error on CPU read access to 0x12345678.
This indicates that an MCECC memory controller detected and corrected a single bit error in one of the DRAM banks. There are a few variations on the message where "CPU" can be replaced with "Peripheral Device" or "Scrubber", and "read" can be substituted for "write". This message is followed by some more details which can help pin-point the error...
memc0: ECC Syndrome 0x23 (DRAM Bank C, Bit#16).
Pin-points exactly where the error occurred.
memc0: Uncorrectable error on CPU read access to 0x12345678.
Errors like this have the potential to corrupt data. As such, it is likely the system will panic very soon afterwards.