The idea of the "base" and the "superstructure" is an example of this. Marx himself barely used the terms. (They come from a throwaway passage in the preface to A Contribution to the Critique of Political Economy.) Rather, they were elaborated after his death by the German and Russian thinkers who codified "Marxism" into a stultifying orthodoxy: Kautsky and Plekhanov, the two thinkers who were the main influences on Lenin, who in turn was the main influence on most people who later called themselves Marxist. The notion that the legal and political institutions of society can be seen as the ephemeral "superstructure" determined by the (really real) "economic base" is actually at odds with Marx's own views.
Although it was Marx's comrade Engels who began the project of codifying "Marxism", towards the end of his life he saw the danger in the drift to orthodoxy, writing to a friend:
According to the materialist conception of history, the ultimately determining element in history is the production and reproduction of real life. More than that neither Marx nor I have ever asserted. Hence if somebody twists this into saying that the economic element is the only determining one, he transforms that proposition into a meaningless abstract senseless phrase.Marx hubristically sought to develop the intellectual tools to calculate, with micrometric precision, changes in the forms of the production and reproduction of life (the complex whirl of movements signalled by the grossly inadequate word "base"). But he never suggested that the patterning of the rest of social life (the so-called "superstructure") could be read off from that. (Apart from in the most general sense, shared with liberals and conservatives, i.e. that parliamentary democracies tend to go with capitalism.)
Or, as the late Norman Geras put it: "between explaining everything and determining nothing, there are real determinants able merely to account for a great deal."
The dogma that "that all human relations are shaped by economics", though it is believed by some stupid self-described Marxists, falls into the category of "crap Marxism" that Marx would have run a mile from.
Previous posts: Arrested materialism; Marxism in an age of waiting; The continued importance of class struggle; Hic Rhodus, hic salta; The necessity and insufficiency of Marxism today; Folk Marxism.