Other mistake: The alien that comes out of the dog is a lot bigger than the actual dog just seconds after it appears. This only applies to the theatrical version, as in the extended version the alien comes out of an ox.

Alien 3 (1992)
Plot summary
Directed by: David Fincher
Starring: Sigourney Weaver, Charles Dance, Charles S. Dutton, Paul McGann
The movie starts right at the end of the second film where Ellen Ripley and survivors start to travel back home. But unfortunately they crash land on some prison planet and Ripley discovers that her companions are already dead because of the Alien. Now the nightmare starts again as the Alien is beginning to kill some of the inhabitants of the prison.
Andrews: We're 25 prisoners in this facility. All double-Y chromos. All thieves, rapists, murderers, child-molesters. All scum. Just because they have taken on religion doesn't make them any less dangerous. I try not to offend their convictions. I don't want to upset the order. I don't want ripples in the water. And I don't want a woman walking around, giving them ideas.
Ripley: I see. For my own personal safety.
Andrews: Exactly.
Trivia: Originally, the movie was going to be shot by cinematographer Jordan Cronenweth, but Cronenweth suffered Parkinson's Disease midway through filming and was let go by the line producer, being replaced by Alex Thomson. However, impressed by Cronenweth's work, David Fincher later worked with Cronenweth's son Jeff in nearly all of his future movies, resulting in two Academy Award nominations.
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Answer: She means that the alien it moves on 4 legs, whereas the other aliens moved primarily on two legs. This alien came out of a dog in the theatrical version and from an ox in the Assembly cut version, which can only move on 4 legs. This hints to the fact that the alien inherits the characteristics of its host.
XIII