...computers. It's even cheaper and better than they imagined; they had no idea how ubiquitous and pervasive it would be.
...travel to low Earth orbit. Sure, you can do it, but the resources it takes require a whole country behind it. And not one of those little countries, either.
...videophones. Been possible for a while, and everyone thought it would be ubiquitous, but nobody does it. Er, no, wait, now that it's been boring for a few decades, it's actually catching on a bit and nobody's noticing it's science fictional.
...airplanes that fly by flapping their wings. Sure, you could do it that way, if you wanted to show off and be pedantically biomemetic, but why would you want to? It wouldn't work very well, and there are far better ways to fly.
...flying cars. Who needs a flying car? They're completely impractical, and besides, we have telecommuting. The problem it's solving is obsolete.
...ballistic travel to the moon. We can get there, but not that way. That way just turns people into a pulpy disturbing mess.
...hypersonic New-York-to-Paris letter couriers. It completely misunderstands the problem, and is horrendously impractical. Solving the actual problem is trivial, and involves completely different technology altogether.
...psionics. You actually thought it was possible? Hah!
...nuclear power. Funny, it turns out to be a bit dangerous. Not really a home appliance after all. Nice technology, though.
...robots that walk like humans. It turns out to be more of a parlor trick than anything useful. It doesn't at all imply the technological abilities you thought it did.
...artificial intelligence. Half of it is still impossible, and half of it is now trivial so we've redefined it as only meaning the impossible stuff, and forget that the trivial parts are ubiquitous and run the world.
...cats. Nobody understands cats. They just are.
...this other thing which I will explain in a comment.