I thought Java applets (those things you'd embed in a webpage) were hot stuff that would completely reshape the Web someday, but they managed to effectively die before the proprietary Flash. I thought Java as a server language was doomed for performance reasons, but that turned out to be just fine. Basically I completely misunderstood where Java was going.
The expectations around smart home stuff were way off too. E.g. expectations were more along the lines of your house reminding you to go buy toilet paper. What we got was the ability to ask a hockey puck to buy some more when we notice ourselves running low, if you can get past the privacy concerns.