Once, while studying Maseches Chullin, Hagaon R’ Chaim Kanievsky Zt”l came across a sugyah related to chagovim (grasshoppers). To better understand the Poskim's perspective, Rav Chaim wished to see a specific type of grasshopper. He asked his daughter to bring one to him, but she couldn't find one.
After resuming his studies, to his surprise, a grasshopper came through the window and landed on his Gemara. Rav Chaim examined it closely and let it go. Later, he realized that he needed to study the hind legs of the grasshopper, but it was long gone. As he was closing his Gemara, another grasshopper hopped onto it, allowing him to study its hind leg in detail.