At the very beginning of Alien you can see the computer after a pan of the ship. The ships computer turns on displaying some random data. There is no mug to the right of the keyboard. After a quick look to the helmet facing the computer it pans back to the computer displaying a tupperware mug which wasn't there before.

Also, when crew member Kane's chest explodes at the birth of the Alien on their dinner table, the first sign of blood at the bursting, is sprayed on. You'll probably have to pause advance to see it but it looks quite stupid replayed over and over again.