I grew up in the Washington D.C. area. I grew up a Redskins fan (whose name I DO believe should be changed). That also meant I grew up hating the Cowboys. My family (and extended family) had season tickets and I could go to 2 - 8 games a year. When not at the games I would watch them at home. I was almost religious in my dedication. I could tell you the names of almost every starter and a good portion of the bench.
I moved to Seattle 10 years ago and now I couldn't name 5 people who play for the Redskins. I'm not quite as fanatic a Seahawks fan as I was a Redskins fan, but I watch every game, go to as many as I can, and can tell you their record, their standing in the league and who the key players are.
Did I set out to become a Seahawks fan and not a Redskins fan? No, I just like football and fell pray to convenience and repetition. Not having Redskins games readily available made it hard to stay up with the current happenings. Not watching games each week I wasn't able to learn what the team was doing well today, learning where their defense was weak and their offense strong. Essentially, I was spending time unlearning what I knew about the Redskins.
The same is true in the software industry. You're only going to know the ins and outs of platforms, frameworks, and code that you actually spend time with. If you don't spend time with it you'll evolve and change on separate paths.
Given enough time, eventually you'll grow apart from the technology. You'll stop recognizing the subtle changes from version to version. You'll stop being able to speak to the differentiation of the platform or framework. You'll make stupid, or novice mistakes when you do dive in. You'll miss important aspects of design, architecture, or code quality.
This isn't always a bad thing. Sometime, in order to learn something new, you need to unlearn something you knew well. But it's important to recognize that if you're not actively learning and growing with a platform, framework, or code base, then you're actively unlearning it.
What are you un-learning today that you shouldn't be?