The detrimental effect that wearing a shatnez garment has on our tefillos is illustrated by the amazing story about the Mirer Yeshiva bochur who left the Beis Hamidrash on Yom Kippur to change his suit.
During World War II, the Mirer Yeshiva escaped to Shanghai and remained there until the war was over. In the midst of the Yom Kippur davening, one of the Mirer Yeshiva students aroused much amazement and concern when he abruptly left the Bais Hamidrash and returned wearing his everyday suit. However, the intensity of the davening pushed all questioning thoughts aside until Yom Kippur ended.
After the fast, when he was questioned about his peculiar behavior, the student simply responded that he did not feel his tefillos were rising to Shomayim (Heaven). No matter how much he concentrated on the words, he just could not maintain proper kavana of focusing on the meaning of the words and feeling the awesome Yom Kippur spirit. Somehow, his tefillos did not seem sincere.
Thoroughly frightened, he tried numerous methods to regain his proper kavana but all were of no avail. Suddenly, a brainstorm entered his mind. He recalled once seeing in a sefer that wearing shatnez can deter one’s tefillos from ascending to the Heavens and perhaps his new suit which was checked, still contained some shatnez. With that thought, he immediately left the Beis Hamidrash and changed into the old suit he had brought from Lithuania. To his relief and surprise, upon resuming his davening, he experienced a surge of spiritual uplifting and was able to daven fervently for the rest of the day.
Shortly after Yom Kippur, the student brought his suit to an expert shatnez tester who discovered that the suit contained linen and wool. The tailor who originally checked the suit was unable to identify the shatnez because the natural linen had been refined and was only discernible to a skilled tester.
The student’s determination to daven with the proper kavana caused him to remember that shatnez can prevent one’s tefillos from reaching Shamayim, and saved him from further transgressing the severe aveira of Shatnez.