Revealing mistake: When the huge alien ship settles over the White House there is an exterior wideshot of the grounds. At the bottom of this shot it's funny how there are many tourists - adults and children, just milling around in front of the gate and across the street with a van driving along between the two sides, and not one has any sort of reaction to the alien ship. This real shot of the White House (some shots had smaller-scale models) had the CG ship edited in during post production. (Visible on fullscreen DVD.)
Revealing mistake: When Stan falls into the dug-out at the baseball game you can clearly see the crash mat that he lands on.
Revealing mistake: When Barb is in the bubble bath, you can see the strap from a G-string she is wearing.
Revealing mistake: After the Martian who is dressed up as a women spits a finger in the fish tank a bit of the Martian's mask near its mouth is torn away. As she chews the nitrogen gum the human lips move but her real teeth don't. (01:04:20)
Revealing mistake: After the second movie sign, when everyone gets into the theater and sits down, there's a very obvious cut, as all the characters cut immediately from the position that they sat down in to a completely different position. (00:28:05)
Revealing mistake: It's very clear they blew up a model of the buildings. The grass seen just before the explosions is way too long and is not as long when the characters are running away. Further, the truck and the barrels have disappeared at the explosion shot.
Revealing mistake: When Le Merchant is offended by his wife's comments and puts the Lament Box into the travel chest, a section of the gold veneer on it has bent upwards.
Revealing mistake: Painfully obvious bluescreen/CGI effect when Snake makes the motorcycle jump into the back of the pickup truck in Cuervo's motorcade. (00:32:45 - 01:08:20)
Revealing mistake: In the deflector dish scene when Worf chops the Borg's arm off with his mek'leth, the blade obviously doesn't even touch the Borg. (01:09:45)
Revealing mistake: A big deal is made of the fact that the female marine is left-handed, but the shot where she throws the knife to establish this fact is flipped - the writing on the poster that she is using as a target is backwards.
Revealing mistake: When Doug 2 wakes up and sees Doug 1, Michael Keaton's body double is noticeable in the angle shot from behind - he has bulkier hair.
Revealing mistake: When Buddy throws Reggie at the piano, you can see it's his stunt double instead of Buddy.
Revealing mistake: When the Leprechaun gets sucked into space, he grabs onto the edges of the cargo bay's doors. You can see pieces of his hands disappear into the doors, showing that it's CG.
Revealing mistake: When Kiki's legs "reverse" at the end of the film as he runs off, watch closely. It is clear the effect was achieved through some shoddy image-manipulation. His clothing and hands "warp" slightly a few times around his legs.