Sorry to go back to the Special Order 937 debate, but since I've read everyone's interpretation of it, i felt compelled to offer my own;
Firstly, in my opinion, there is no doubt whatsoever that Ash knew about the special order BEFORE he left Thedus, he was put on the ship SPECIFICALLY for this task, and the company (or at least whoever issued the order) was aware of the hostile organism BEFORE the Nostromo landed on LV-426. Now, before you write me off, here is the evidence presented in the movie;
1. The most senior officer is the only crew member on the ship with access to Muther. During the opening scenes of the movie Kane is heard saying "Dallas, Muther wants to talk with you" to which the captain responds "Yeah...my eyes only." Later in the movie, after Dallas is (presumed) dead (or killed depending which cut of the movie you roll with) and Ash asks Ripley what she wants him to do, she responds "What you've been doing; Nothing. I have access to Muther now and I'll get my own answers thank you." (Implying she didn't have access prior to Dallas' demise, and, as she stated earlier in the movie "When Dallas is off the ship, I'm senior officer." Now...If this was all a co-incidence, and Ash didn't know anything about this before they left Thedus, then, explain how he received special order 937 once the Nostromo had left for Earth? He had no access to Muther. Thus, he had his orders before they left Thedus.
2. The company (or whoever issued special order 937) knew in advance there was a hostile organism on the planet. Whether that be prior knowledge due to a previously unseen encounter, or (most likely) the Derelict warning beacon had already been deciphered fully. There is no chance that (as someone suggested here) they *didnt* know until Kane was facehugged. How do we know this? Again, because it is in Special Order 937; (see highlighted sentence in screenshot)
So what does this tell us? They knew there was a hostile organism on the planet before the Nostromo got there... Dallas states that he shipped out 5 times with another science officer, who was replaced two days before they left Thedus with Ash. Ash would have HAD to be aware of his orders before he left Thedus (meaning he was put there intentionally) In my opinion, i can't see how or why any of this is being debated...its all in the movie