If a DIMM mismatch error (size, rank, or speed) occurs on odd-numbered DIMM pairs, it might cause a ddr-training-failed error on the entire branch, thus disabling all DIMMs on that branch.
Furthermore, Oracle ILOM might not mark them as faulty or send a diagnostic message, but only send one ereport on boot:
Use the Oracle ILOM Fault Manager to view information about active faults (fmadm faulty) to identify the mismatched DIMM pair.
Then replace the pair with a matched pair. If the fault isn't cleared automatically, you must manually clear it using the appropriate Oracle ILOM fmadm repair command as described in the Oracle ILOM documentation.