UPDATE: In an exclusive interview with ET, Shannon Beador details the altercation between her and her husband's former mistress, Nicole McMackin, and adds, "I never touched that woman." See the full interview here.
A fight went down at the University of Southern California, but it wasn't on the football field.
Real Housewives of Orange County's Shannon Beador got into an argument with Nicole McMackin -- her husband David's former mistress -- at the USC-University of Utah football game at the Los Angeles Memorial Coliseum on Saturday, an eyewitness tells ET.
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Both Shannon and Nicole's husbands are USC alumni, and the two couples have frequently been seen at Trojans games. An eyewitness tells ET that on Saturday, in "an unfortunate coincidence," Nicole and her husband's seats were only about fifteen rows in front of where Shannon and David were seated. Nicole's husband has season tickets.
According to the eyewitness, Shannon began yelling while the two teams were doing warmups on the field. Then, at the beginning of the second half, the argument erupted when Nicole and a group of her friends were walking through a tunnel to return to their seats. Shannon, who was standing at the side of the tunnel, reportedly started yelling and swearing.
Nicole's husband jumped in, saying, "Shannon, go sit down. No money to be made here. Please let us be," according to the eyewitness. One of Nicole's friends then started yelling, threatening that if Shannon didn't leave them alone, she was going to call security.
WATCH: 'RHOC' Star Shannon Beador Reveals Husband David Left Her During Affair
Shannon answered by grabbing Nicole's arm, and Nicole released herself by flinging her arm back, the source adds.
"Wait until Monday. Wait until Monday. The press will be all over you again -- you just wait," Shannon reportedly yelled to Nicole.
Shannon is of course referring to part two of the RHOC reunion show that airs Monday on Bravo.
Later in the game, when Nicole and her friends were walking down the stairs, Shannon started jumping up and down, screaming, "That's Nicole McMakin. She slept with my husband about three to four times." David, embarrassed by it all, sat in silence with his head down, according to the eyewitness.
That wasn't the end of it, however. Shannon took to Instagram after the game to throw even more shade at Nicole.
WATCH: Shannon Beador Talks Fixing Her Marriage: 'We're In Such A Happy Place'
"Go Trojans! Cheering with good friends! Sorry @tamrajudge couldn't make it! Someone with no moral compass decided to show up again. Watch reunion part 2 on Monday night and you will understand who I'm talking about. #narcissist #stalker," she captioned a photo of her friends at the game.
Later, she posted a photo of her kissing David on the cheek.
"Love my husband," she wrote. "Having an amazing night at the most incredible restaurant @panxacocina #restored #godisgood."
David's cheating scandal first came to light on an episode of RHOC that aired in June, where it was revealed that he cheated on Shannon with Nicole for eight months. Shannon also revealed that during the duration of the affair, there was a two week period where David actually left her.
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"I don't think that people understand the level of hurt," she said on a RHOC reunion show earlier this month. "When David left me for those two weeks, he would still come and pick the kids up from school. One day he handed [our daughter] Stella his wedding ring, and said, 'Give this to your mother. I don't want it anymore.' That was a knife in my heart. I thought, 'We're never going to get back together.'"
As for Nicole, she and her husband have reportedly repaired their marriage, and a source tells ET that she knows she made a mistake and they just want to move on from the incident.
ET caught up with Shannon back in July, where she talked about bouncing back from her husband's affair. Watch the video below to hear more.