Over Pesach I heard this story directly from the Levi.
He had travelled to receive Brachos from the Lubavitcher Rebbe ז’ל and to bring his son around the time of his Bar Mitzvah. He is a Levi and was called up as a Levi in 770. The next Aliyah, Shlishi, went to the Lubavitcher Rebbe. The Rebbe faltered and instead of starting with ברכו he began the ברכה of אשר בחר בנו. Nobody said anything to correct or interrupt the ברכה. At the end of the Aliyah, after the Rebbe said the second ברכה of אשר נתן לנו, he then said ברכו. Of course, one can say ברכו at any time and have ten people answering.
Upon returning to Melbourne, the Levi mentioned this story to members of the Chabad Kollel. The reaction was
“You’ve misunderstood. The Rebbe did it on purpose. He wanted to teach people what the Halacha was”
Unfortunately, these were also very high quality אבריכים from the USA (from several years ago). It’s somewhat sad that they couldn’t see the Rebbe as a human being, as well as a great צדיק and מנהיג ישראל (or נשיא)