Brad DeLong on one fatal miscalculation by Marx and Engels:
The workers are supposed to organize, lobby, vote, agitate, and strike for higher wages, better working conditions, more political voice, government regulation of industry, and an egalitarian distribution of income and wealth. The struggle will weld them into a single conscious movement. And they are supposed to lose. And when they lose--so Marx's and Engels's plan for world history went--they will recognize that the system cannot be successfully managed or changed or ameliorated but must be overthrown, be transformed, be superseded.
But what if they did not lose but won instead? And what if those victories did not strengthen but weakened working-class consciousness as a single group with common interests? And what if those victories convinced more and more peopke that the system could be worked within, and did not need to be overthrown? What then?
By the time you are thinking like Engels was thinking in the early 1890s you are well on the road to Lenin and Stalin...
There are extensive quotes at the link.
It's not their only miscalculation, of course. But it's a big one, perhaps even bigger than the miscalculation that the merchant class would join the proletariat in overthrowing the bourgeoisie, or that capitalism would rot from the inside out rather than rejuvenate itself through creative destruction. Their fundamental misunderstanding of democratic politics renders the rest of their political economy moot. And it's an under-discussed miscalculation: much attention is paid to the internal contradictions of collectivist economics, but less to those of collectivist politics.
As DeLong notes, the world was changing all around Marx and Engels in the 1880s and 1890s. But despite the fact that Marxism is praised as a dynamic theory, the dynamics are all wrong: the world was moving in a different direction, and they were unable to recognize it. Too bad.