I don't think many people have noticed, probably because the quality of Google's search results has remained so high, but the way Google handles its search input has changed several times over the years. A somewhat recent (and possibly gradual) change was the way that Google now automatically includes various forms of words. For example, a search for the word testing will include the word test, and a search for the word test will include the word tests. It used to be that you had to search for test OR tests if you wanted Google to search for either word. Now if you want Google to exclude variations of a word, you have to use a plus sign or quotes (+test or "test").

One quirky thing that's been going on for years is the way that adding certain words to the search input can lead to a greater number of results. For example, "a search for kabul OR kaboul OR kaboel found [fewer] results than just kabul OR kaboul even though it should have found more." I'm fairly certain I've seen this occur even without the use of the OR.

This post about the future of Google is interesting...