In the scene where Wells asks Cooper to knock him out, Kevin McKidd (Cooper) throws a stage punch the first time, but misjudges the distance of the second and catches Sean Pertwee (Wells) on the nose. Pertwee didn't feel the punch however as he really was drunk for that scene.
When the squad first lands from the helicopter, Sgt. Wells tells Cooper "Get a position and bearing. I want to be on the move in three minutes." He gives the command for the squad to move out exactly 3 minutes later.
The piece that Megan plays on the piano halfway through the film is Debussy's "Clair de Lune", roughly translatable as "moonbeam". The link is obvious, but is possibly also a nod to An American Werewolf in London (1981), the soundtrack of which consisted purely of songs with "moon" in the title.
There is very little CGI used in the movie because the people involved in the filming believed that CGI was being over-used at the time and that it would take viewers out of the movie because they would be focused on how the special effects looked rather than the story, thus the werewolves are animatronics and body suits with stilts.
The film makes several references to Zulu (1964). There's the choral music featured in Zulu when Spoon is talking about Rorkes drift, and "Dog Soldiers'" Sgt. Well's paraphrases "Zulu's" Colour Sgt. Bourne's "be quiet now will you, there's a good gentleman, you'll upset the lads" when talking to Ryan.
One of the soldiers in this movie is called Bruce Campbell, a reference to The Evil Dead (1981) (Bruce Campbell is the actor who portrayed its hero Ash and the film seems to have partially inspired the plot of Dog Soldiers (2002)).
The insurance did not cover the actors jumping out of the helicopter early in the film. As most of the crew were ex-army they jumped out of it instead. The crew also doubled up as Sgt Well's soldiers for some of the tabbing shots.
Another reference/homage to Zulu (1964) is when Cooper's character is trapped in the bathroom upstairs and he begins tunneling through the wall to get to Wells. The two then proceed to break through the wall to the bedroom, mirroring the scene in Zulu when Hook is in the Hospital as the Zulus break in.
The trivia items below may give away important plot points.
Being Scottish, Kevin McKidd who plays Cooper spotted that there is nowhere in Scotland that is a four hour drive from anywhere as mentioned by Megan in the film, but chose not to say anything. This line can be taken as an example of "unreliable narrator" since we later find out that she is also lying about where she is from and how close the next house is.
Originally Spoon's on screen death was being ripped in two by the werewolf but it was cut to what we see in the film because Neil Marshall didn't think that audiences would be able to handle such a graphic death to such a well loved character.