That's one that grate on me. When we're very young, we might say, "It's me;" or "Me and him did ..." We're taught that the proper way is "It's I;" or "He and I ..." Somehow, that morphs into the misconception that the nominative case is to be used in all situations. Even people supposedly educated in language, such as news anchors, will do this.
One I struggle with is the difference between "affect" and "effect". I stop and ponder whenever I need one or the other, and I'm never confident I selected the proper word.