Ferris Bueller makes an appearance from afar, also from Hughes and Paramount. at first it looks like a clip from the movie cause it cuts away, but then shortly after closer to the camera he says "Jim"

also bill murry ~ on the movie set of Scrooge

If you notice in a scene from "Planes, Trains and Automobiles" where Steve Martin's wife is watching TV, you can here a scene from "She's Having A Baby" on the TV that she is watching.

The movie the Great Outdoors and Planes TRains and Automobiles were all being shot about the same time. John Hughes wrote all the scripts so the other actors came by.

At the end of "She's Having a Baby" (during the credits I think) there are a series of clips of people suggesting baby names. Some look like people being interviewed on the street while others are cameos by actors & actresses. John Candy & Dan Ackroyd both have clips. You can tell that they are on the set of "The Great Outdoors" and some of the other actors from the movie have clips too. There's only a 4 month difference in release date between the 2 movies. I assume that either they were being filmed at the same time or "The Great Outdoors" was being shot while "She's Having a Baby" was in post-production.