Marx and Engels big mistake was in not realizing that despite the abuse heaped upon them, the powers that be at that time recognized at the very least that the notion of class struggle as a driver of history had at least some merits. Marx fully expected a series of revolutions in the latter half of the 19th century, and in some cases it almost came true, but then suddenly you see several nations, even the Austro-Hungarian Empire, for goodness sake, enacting liberal constitutions. In Britain, in particular, within 20 years of the Communist Manifesto's release, the Reform Act of 1867 greatly expanded the voting franchise, enfranchising a large number of working class members. This inoculated a good deal of Europe against any kind of Socialist Revolution.
What went really wrong for Marx's economic and political theories was that first Communist states were fundamentally agrarian states; Russia and China. These, even by Marx's own theories, were not yet at a point of economic evolution that they should become Communist, and in fact, the Communist rulers of these states, to keep with Marxist ideas of evolution, had to introduce vast industrial programs, almost trying to create a Bourgeois middle class just so they could fulfill the checkboxes on Communist revolution. The industrialized states that became Communist were pretty much the states that the Soviet Union forced into its sphere after the Second World War, and who had initially gained their industrial capacity through fundamentally capitalist means.
No one has ever actually seen a Communist revolution the way Marx foresaw such a revolution happening, mainly because, as I say above, the Western nations, whether intentionally or by accident, liberalized sufficiently that the working classes could join political parties, or form new ones (like the Labour Party in Britain). I like to imagine that Disraeli, crafty fox that he was, was at least partially cognizant of the potential for a revolution if Westminster didn't let at least some of the lower classes in, and it wasn't all about just taking the piss out of Gladstone.