It is possible to see an upside down R2-D2 (from Star Wars (1977), etc.) in part of the large spacecraft that flies over Devil’s Mountain. The SFX people needed more detail, and so supposedly there are many more such items, such as a shark from Jaws (1975) (also directed by Spielberg), etc. R2-D2 is visible as Jillian first sees the mothership up close from her hiding place in the rocks.
The scene is towards the end of the film, after the army has cordoned off the mountain, and just after the hero (played be Richard Drefuss) jumps out the helicopter and runs up the mountain.
Look carefully at the scene where Richard Dreyfuss is peering over the edge, into the crater, and gets ‘Buzzed’ by one of the largest UFOs / spaceships.
On the underneath of the spaceship, fixed upside down, is R2-D2 from Star Wars!
On the DVD, you can see the R2-D2 at time index 1:54:03 to 1:54:05