Here’s a great story from Rabbi Yitzchak Brand. It was related to him by Rabbi Aaron Treger of Antwerp, son-in-law of Rabbi Shlomo Zalman Auerbach:
An Avreich once came to Rabbi Auerbach and happily proclaimed he had learned all maseches Eruvin in depth. Rabbi Auerbach asked: do you carry on Shabbos (relying on the Eruv)?
[Before I continue the story, let me note Rabbi S.Z. Auerbach was not known for opposing modern Eruvin.]The Avreich answered in the affirmative, to which Rabbi Auerbach retorted: if so, you haven’t learned Eruvin…
Source: right here.