I dont know if change of mind is the right word. But may be proven wrong.
JVM. I was fist convinced JVM will be fast enough, and then it will never be fast enough for lots of things, and right now it is being used for even high frequency trading.
Excel is much much more important than most people are aware of it. When I was young I thought Excel / Spreadsheets or Access were toys. But working in the real world and some years passed I have a change of perspective. Since then I have been constantly calling it the most important software in computing. Far more so than even Windows or Linux.
Hardware Matters, a lot more than Software. This seems counterintuitive when most people would consider software is where the value lies. Especially when I started out as a Hardware Engineer but believed what Bill Gate said some 30 years ago, with Moore's Law all hardware cost will trend to zero. In reality Consumers dont understand software. And they are reluctant to buy software or features. While this works differently in Business, consumer tends to prefer something physical. Hence buying a new computer with free software update rather than paying for software. It is the whole software and hardware together as a "Product", and not one or the other.
Internet, or more specifically web pages. Are not what we envisioned with the Internet Super High Way. The actual network, where we can stream Netflix, or Spotify with 5G is great. But information on the web is, for vast majority of cases a net negative.
Mobile or Smartphone Gaming. Out of the 4B smartphone users, close to 2B are gamers. I never thought of a day that everyone is a gamer. In the old days Games were different. It could be an RPG with great novel and story telling. A simulation game like Sim City, Civ or Age of Empire that you could learn history or how the world works. Modern Mobile Games are a Casino, and the free ones are riddled with Ads. With both Smartphone platform owner have ZERO gaming DNA in them, but only a business person of how to squeeze every single penny out of it. Everything you imagined with Mobile Gaming didn't happened. Instead we got a Mobile Casino.
Objective-C - What was once thought to be an ugly hacked on top of C Programming languages. Is actually beautiful.