Sorry if this is common knowledge... but I do not see this posted anywhere.

I have found that Mary Shelley's Frankenstein, Baywatch, and Batman Forever all have a hidden screen.

During attract mode, hit the following flipper combination:
Left x 3, Right x 3, Left x 2, Right x 2, Left, Right

You will be shown a secret screen...

Mary Shelley's Frankenstein has a number and text of "Bottles Of Beer On The Wall"
Baywatch has a picture of a Craw, a number, and text of "Craw Sightings"
Batman Forever has a number and text of "Bottles Of Beer In The Craw"

Anyone know the story behind these? What number does your game show? Is it always 00?

The Easter Eggs for X-Files are a little different...

During X-Files attract mode, hit the following flipper combinations:

(L, R, Lx2, Rx2, Lx3, Rx3) = "Gallons Of Hot Garbage"

(Lx2, Rx2, Lx2, Rx2) = Credits

(Lx4, Rx7, Lx8) = "Good Luck Dave Hegge! Retired From Pinball"