Watch Moment Farrah Abraham Shocked Teen Mom Family Reunion Cast with Surprise Arrival (Exclusive)

"Teen Mom Family Vacation" brings together "Teen Mom OG" stars Amber Portwood, Maci Bookout and Cheyenne Floyd and "Teen Mom 2" stars Leah Messer, Jade Cline, Briana DeJesus and Ashley Jones, for a retreat in San Diego.

Maci and Cheynne told MTV they could invite any present or former cast members they wanted -- and production clearly took them up on that by having Farrah pop up as a late arrival. In this sneak peek clip at tonight's episode, she shows up just as everyone's starting to get along.

"I'm trying to live my best life," exclaims Cory Wharton, trying to get everyone good and drunk following a scary incident involving Briana earlier in the day. "Nobody's kids are here tonight, please enjoy yourself!"

"We're gonna do a toast to Briana getting better. We're gonna do a shot to her recovering from the Cory Challenge," adds Briana's ex Devoin, as everyone joins in for a toast.

"I think we're all gonna have a really good time for the rest of our time here," said Amber, clearly unaware of what's about to happen. "This will be great. Cheer everybody, let's have some fun!"

With that, a "Hi!" is heard from behind them as all the cast members turn around to see Farrah walking toward them. "I feel a demonic presence. It's creeping up on me," jokes Ashley in a confession, while Amber reacts with a simply, "Well, f---."

Of course, any regular "Teen Mom" fans know Amber and Farrah have gotten into it hard in the past. Speaking with TooFab ahead of the season, Portwood opened up a bit about having her touch down in San Diego.

"I sat down with her the first night when she first came in. I didn't really know why she was like here, but it was okay," said Portwood, who has had some explosive moments with Farrah in the past. "We had got done with a really nice toast together -- of course mine was lemonade -- and all of a sudden it's just like a little bit of a surprise. But I got to sit down with her and chillax and talk for a minute."

"I hope I'm not giving too much away. It didn't matter anyway, because there was still drama," she added. "But there were so many other people ... [production] would just throw people at us whenever they felt like it! it could be 11:30 at night or 2 in the afternoon. It was fun though. It was kind of like, no hard feelings because like Maci said, everybody deserved to be there in some way."