The following Hasidic story was sent to me:
One Pesach, the Maggid of Mezritch sent 3 hand-baked Shmurah Matzos to his beloved student, Rabbi Levi Yitzchak of Berditchev, via a shliach (an agent or messenger).”Beware!” he warned the shliach, “these matzos are only for Rav Levi Yitzchak!”The shliach thought to himself, “If only I could merit to have a matzo from the Maggid at my seder!”Then he thought, “Why don’t I just take one matzo for myself? Rav Levi Yitzchak will surely never realize it.” And so he did. He handed over two matzos to the Berditchever and kept one for himself.
Now the Maggid’s matzos were extra thin and well-done charcoal black so as to ensure that they were well-baked and chametz-free. This unique-looking matzo adorned the shliach’s seder table and when the time came for the mitzvah of eating the matzo, he pronounced the blessings and ate a “kezayis” (an olive-sized portion) from the Maggid’s matzo.
No sooner did he attempt to swallow it than he began choking and screaming. He fell to the floor writhing in apparent agony. His poor distressed family tried to get him to cough up the matzo, to no avail. Whatever they did awoke him and then he promptly screamed and fainted off straight away. Eventually, after enlisting the help of neighbors and friends, they called for Rabbi Levi Yitzchak.
“Please explain exactly what happened,” ordered Rav Levi Yitzchak. The wife described how her husband had simply eaten a kezayis matzo and how he choked and screamed and how he kept fainting. “Describe the matzo please.” When she described the black charcoaled thin matzo, Rav Levi Yitzchak’s features turned grave, “Oy Vey! If he ate from my special matzos he is in grave danger indeed!” “Please, Rebbe, save my husband,” the tearful woman pleaded.
“Here,” he gave her a piece of maror (bitter herb), “Give him a piece of my maror.” When she did, he promptly coughed up the kezayis matzo and returned to himself. Once he had somewhat recovered, Rav Levi Yitzchak asked the poor Jew what had transpired? “Oy Rebbe, I am so sorry! I took your matzo. Now I understand the Maggid’s warnings!!! For as soon as I ate from your matzo, the room filled with the entire retinue of the heavenly beings, all manner of fiery angels and seraphs descended and all the heavens opened up and I saw their myriads hosts! I was seized by such fright that I fainted. Each time my family revived me, I saw them and fainted again, until I coughed up the matzo.”
And now we know what Rav Levi Yitzchak of Berditchev experienced when he ate the Maggid’s matzo! 
Some of my own thoughts:
1 — Although in this re-telling, it’s the particular stringency with which the matzo is baked that gives it the quality to impart visions, in other Hasidic stories (and in other versions of this one), it’s the kavannah, the “spiritual intention” during the preparation and baking that makes the difference. It’s also reflective of the spiritual purity of the person baking it – in this case, the Maggid. The Maggid’s purity purifies whoever eats the matzo he prepares.
2 — Of all the elements at a seder, why was “maror” the “healing” element here? “Maror” — usually horseradish — can be startlingly bitter. There are milder, even sweet elements at a seder, too. Maybe the “maror” was meant to shock the erring shliach’s awareness, like “spiritual smelling salts,” and bring him back down to physical reality? But if the “spiritual purity” of the matzo is what purified his own vision, then maybe the “maror” represents a taste that he rejects. In that rejection, his self-will brings about enough spiritual impurity to “revive” him. Also — Rabbi Levi Yitzchak says that the “maror” is “his” — i.e. from his own seder plate. But he had left his own home to come to the shliach’s. How could the “maror” be “his”?
3 — If the shliach was supposed to deliver 3 matzos, wouldn’t Rabbi Levi Yitzchak have noticed when he received only 2? In another version of the story, the shliach substitutes one of the matzos. But, then, wouldn’t Rabbi Levi Yitzchak notice the difference between the other 2 unique matzos (“extra thin and well-done charcoal black”), and the “substitute?”
4 — Wasn’t Rebbe Levi Yitzhak having his own seder as all this was happening? Wouldn’t he have noticed the spiritual difference between the matzo he was eating and the one he was supposed to receive?
5 — The Maggid’s association with the preparation of the matzos gave them a particularly purifying quality. There’s a similar story about the Maggid himself: Once, an infertile couple came to him, asking that he pray that they have a child. The Maggid asked the woman to hold his gartel — his belt — while he prayed. When he finished, he asked the woman if she’d felt anything. She said “No.” He prayed again, asking her to hold his gartel. Again, she said she felt nothing. Then, he did so a third time and she thanked him. On the way home, she told her husband that the first time the Maggid prayed, she was filled with a spiritual ecstacy that she thought she’d never feel again. So, she told the Maggid she didn’t feel anything and felt it again the 2nd and 3rd time, after which she couldn’t pretend any further. Soon after, the couple conceived.