Revealing mistake: Right after they "create" Leeloo and she is lying in the plastic cage with the little straps on her, the guy approaches her and says, "if you want out, you are going to have to work on your communication skills." She gives him a dirty look, and then you can see a huge hole, pre-cut in the glass, that she busts her hand through.
Revealing mistake: Whilst Diva Plavalaguna is singing, you can see the back of her head creasing and bending. This is because the suit that the actress is wearing is made out of foam latex; confirmed by Nick Dudman, Creature Design Supervisor on the DVD featurette "An Audience With Diva Plavalaguna".
Revealing mistake: After Korban meets Ruby and everyone leaves them alone, Korban grabs Ruby by the neck and holds him to where his feet can't even reach the ground. In the first shot, you can tell that Korban's hand isn't even applying pressure. In the next few shots of Ruby, still being held off the ground, you can now see that Korban's applying a little pressure now, but probably not as much as there should be while holding a full grown man a foot off the ground.