Vitamin B12 toxicity or excess: Elevated B12 levels can be due to excessive intake or supplementation, which may cause neurological symptoms, including memory problems, although this is less common.

Polycythemia vera: A myeloproliferative disorder that can lead to elevated B12 levels due to increased production of transcobalamin I and III. Memory problems can occur due to hyperviscosity or thrombotic events.
Chronic myeloid leukemia (CML): Similar to polycythemia vera, CML can cause elevated B12 levels. Cognitive symptoms, including memory issues, can be present due to the disease itself or its treatment.
Liver disease: Certain liver conditions can lead to elevated B12 levels. Liver dysfunction can also cause cognitive impairments, including memory problems, due to the buildup of toxins.

Leukemia (other types): While less common than CML, other leukemias can also lead to elevated B12 levels and cognitive symptoms, including memory problems. Missing a diagnosis of leukemia could be fatal.
Malignancy (solid tumors): Some solid tumors can produce transcobalamin, leading to elevated B12 levels. Cognitive symptoms can occur due to paraneoplastic syndromes or direct tumor effects.
Vitamin B12-dependent neurological disorders: Although less likely with elevated B12 levels, certain conditions like transcobalamin deficiency or abnormalities in B12 metabolism could lead to neurological symptoms despite high B12 levels.

Transcobalamin II deficiency: A rare genetic disorder affecting B12 transport and utilization, potentially leading to neurological symptoms despite elevated B12 levels.
Methylmalonic acidemia: A disorder of amino acid metabolism that can lead to elevated B12 levels and neurological symptoms, including cognitive impairments.
Folate receptor autoantibody syndrome: A rare condition where autoantibodies against folate receptors can mimic or cause B12 deficiency symptoms, potentially leading to memory problems, despite elevated B12 levels.