If you're familiar with Marx's work beyond capitalism-bad-communism-good, I think they're fairly obvious, but I'll bite since most of the other comments so far are fairly brief quips. Marx predicted that communism would rise from the internal collapse (or revolution, but the two are interchangeable in his context) of late capitalist societies, however this has not happened. Neither China nor Russia were really capitalist societies in any reasonable sense when they turned to communism, if anything, you could call them agrarian.

Similarly, Marx claimed that as capitalist societies develop, workers would face increasing poverty, declining living conditions, etc. While the income gap between the top and low end is ever increasing, in the grand scheme of things, living standards for workers have improved considerably across the board in pretty much every category. Marx also argued that capital accumulation would lead to a decrease in profit margins, which would eventually result in an economic collapse, again ending capitalism. While we've seen numerous economic downturns, practically all affected economies have eventually recovered.

And so forth, there's many other examples we can take turns picking apart, but I doubt there's much value to it — most people arguing over Marx don't actually know much about Marx.