The Rebbe Rashab was known to boil before Pesach all sugar he consumed during the Yom Tov. The idea was that as the sugar dissolved into liquid, any wheat kernels in it would become noticeable and would be removed. (The sugar back then was not necessarily as pure as it is today and there were legitimate concerns of other grains mixed in it.)
His own brother-in -law R’ Moshe Horenstein (I’m not sure he was a Lubavitcher) owned a sugar plant. During one Pesach he personally offered the Rebbe some cubes vouching that his factory went to great lengths to ensure all sugar was pure. The Rebbe turned one cube on all its corners. Then, breaking the cube, a seed came right out.
R’ Moishe later remarked that this incident did not sway him. He reminded his listeners about a מחלוקת the holy Arizal had with the local מרא דאתרא, meaning the Bais Yosef about a specific animal part. The Beis Yosef writes in Shulchon Aruch that the part may be consumed after all blood veins and fats were properly removed (or cut open). The Arizal, however, maintained that the part is too infested with blood veins and fats and cannot, therefore be properly cleaned of these items.
At a repast he hosted, the Beis Yosef served the Arizal the exact part they were debating. He wished to finally bring the matter to rest. The Arizal simply turned over the meat a few times, cut it open and showed the Bais Yosef some veins which were overlooked. The Beis Yosef was very disturbed by this incident. He practically advised world Jewry to eat a meat cut which was certain to contain prohibited blood and fat. In his dream that night, however, the Beis Yosef was advised not to be discouraged. The Arizal in his great holiness could produce blood veins on the walls…
“So too,” concluded R’ Moshe, “the Rebbe can produce grains on the walls…”
(Rav Zevin relates a similar story regarding R’ Akiva Eiger and his son-in-law the Chasam Sofer.)