In Russia, during the time of the First World War, anyone who was caught without the proper identification papers was suspected of being a spy, and was subject to interrogation, imprisonment, or worse. On the day the Chazon Ish completed his commentary on Meseches Eruvin, he was sitting in the shul in Minsk when suddenly someone entered and announced that a group of soldiers had gathered on the outskirts of the city, and were preparing to make a search for all those who did not have their papers in order. Since the Chazon Ish did not have the necessary papers, he fled to the forests outside the city to hide until the soldiers completed their search and left. When he reached the edge of the forest, he realized that he had walked right into the soldiers’ camp where they were preparing to enter the city. Since he was already in clear view of the solders, he could not turn around and run. He had no choice but to walk straight through their camp, trying to appear as innocent as possible. Amazingly, the soldiers took no notice of him at all, and he walked safely through the forest.
The Chazon Ish concluded that Hashem had preformed this great miracle for him in the merit of his studies in Meseches Eruvin.
(Translated from Toldos Yaakov, p. 108, by Meorot HaDaf Ha Yomi, vol. 347; see also Hamodia Magazine, August 30 2006, p. 10, for a slightly different version of the story)