Oprah Winfrey Says 'Ugly Cry' During Mary Tyler Moore Surprise Led to This Rule on Set (Exclusive)

When Mary Tyler Moore emerged from backstage as a surprise guest on The Oprah Winfrey Show in 1997, Winfrey immediately dissolved into tears. Moore was one of her longtime role models, and the talk show host was left all but speechless on-air.

"I went into the ugly cry," she tells PEOPLE in an interview celebrating the magazine's 50th anniversary.

Never again.

"I said, nobody will ever surprise me again [and] if you want to continue to work here, do not surprise me!" she recalls, with a laugh.

The lone exception came towards the end of the hit show's 25-year run in May 2011, when Winfrey, 70, gave the green light to producers to prep what would be a star-studded homage and a night of surprises for the iconic host.

"The show at the United Center in Chicago was the only show that I did not have a pre-show meeting about, and the only show that I ever allowed my producers to do something and surprise me," she says of the occasion that saw a veritable galaxy of celebrities, including Tom Hanks, Beyonce and Aretha Franklin, descend on Chicago to pay tribute.

"The [producers] actually came to me and said, 'Can we please surprise you on this night?' And I let go on that night," Winfrey recalls. "I didn't know anything that was happening... I just walked into the stadium and the whole entire evening was a surprise to me."

For more from PEOPLE's 50th anniversary, pick up this week's issue, on newsstands Friday.

While tears were indeed shed, the experience was a huge and meaningful success. Winfrey says: "I just loved that night so much."