It is not always easy to prove modus operandi because behavior discovered must be unique. Saying, "Well, he carried a gun when he robbed the bank," isn't uncommon and wouldn't be much of a strong M.o. On the other hand, if the criminal carried a gun while sporting a rubber duck on his head, the M,O. gets a little more interesting. Usually it takes several distinctive properties to make a strong case that a person may have committed a number of crimes of the same nature.