The phenomenon of learning something new (in particular a word) and then realising it was something in your environment all the time. In other words, things that have been around a while, just you haven’t noticed them. Common among foreign language learners.
Origins: a play on serendipity, the old word for Sri Lanka. Diego Garcia is another island in the Indian Ocean. First coined on by user aldiboronti on wordorigins.org and has since spread, having been featured as word of the day on several sites.
Funny you should mention diegogarcity. I was just thinking about that this weekend. I recently got a new car (new to me, that is), and now, I suddenly notice how many of the same model are on the road. And here I thought nobody drove Buicks any longer.