While a catastrophic failure *could* theoretically cause some mixing (e.g., a massive crack in the cooler simultaneously breaching both oil and coolant passages), this is extremely unlikely. A more common scenario would be a coolant leak into the engine block (head gasket failure, cracked block) which would then contaminate the oil, but this isn't a failure of the oil cooler itself. The oil cooler itself would more likely leak oil externally if it fails.
