The fame of the great Torah scholar Rabbi Avraham Yeshayahu Karelitz (the Chazon Ish) preceded his visit to the Russian city of Minsk, but few people actually recognized him in the early years.
When he entered a local synagogue and took a volume of the Talmud from a shelf for study, he could not know that a shiur was soon to be given to a group in that very Gemara. The gabbai snatched the Gemara from his hand angrily saying, "A simple Jew should be reciting the psalms of Tehillim; the Gemara is needed for the participants in the shiur!"
The next morning the gabbai approached him to ask his name in order to call him for making a blessing on the Torah. When he heard his name he realized that he had insulted a Torah giant who would someday be the leader of his generation and he profusely apologized for his behavior.
In his characteristic humility the Chazon Ish gently responded:
"Everything you said was perfectly correct. The people in the shiur had a priority to the Gemara I was holding, and a simple Jew should recite Tehillim."