Twin Pines Mall became Lone Pine Mall after Marty changed the future in “Back to the Future” (1985). Is that an Easter Egg or a Thing in the Movie? Let’s find out!
To me, an Easter egg is a fun gag referencing something that doesn't require full audience understanding to decode the character or plot point. For example, the Harold Lloyd "Safety Last!" clock operates as both an Easter egg and a setup+payoff gag.