17 When a Fight Club member sprays the priest with a hose, the camera shakes briefly. This is because the cameraman couldn’t stop himself from laughing.

If you watch the clip above with concentration, you will see that there’s a tiny shake when the Fight Club member sprays water on the priest. Well, the reason is during the filming of the scene, the cameraman couldn’t control his laughter! (source)

18 The scene where Norton is grinning at the board meeting and blood fills his mouth was actually an accidental shot.

There is a scene in the movie where the narrator is sitting at a board meeting on a Monday morning. When he opens his mouth, there is blood all around his mouth and his colleagues are horrified. This close-up shot of the narrator happened between takes. The director David Fincher asked Norton to show him how much blood was there in his mouth. Norton looked at the director and opened his mouth. The cameras were rolling at that time and captured this moment. Fincher liked it and put this accidental shot in the movie. (source)

19 When the narrator is sitting at work writing Haiku poems, the names on the document on his screen are the names of the production assistants and crew members that worked in the movie.

In the scene where the narrator is writing Haiku poems and says that he had been mailing them to his colleagues, there is an email list open on his screen. The names on the list are actually the names of the crew members and production assistants. (source)

20 All the buildings that blow up at the end are real buildings that have been altered by using CGI effects.

At the end of the movie, the narrator and Marla stand together and look through the glass at the crumbling buildings outside. That scene, of course, isn’t real. No buildings were actually harmed during the filming of the scene, but the buildings portrayed were real.

The buildings were real and they exist today as well. But the view that is shown through the glass is not real. The “view” is actually a studio composite of ten different buildings (primarily from Century City and Los Angeles), which were rearranged from their original location arrangement. After this was done, CGI effects were used to bring them down in the last scene.

The last two buildings to fall are the twin Century Plaza Towers. To the left is the Fox Plaza Tower and to the right is the St. Regis Hotel tower. In reality, these buildings are located in totally different places. Also, most of the buildings depicted are owned by Fox and is a showcase of the healthy relationship between Fox and the director David Fincher. (1,2)

21 While opening the menu of the Blu-Ray version of Fight Club, a menu pops up of the film Never Been Kissed for a few seconds.

When you first open the menu of the Blu-ray version of Fight Club, you may end up thinking for a second that you brought the wrong DVD. This is because instead of the Fight Club version, they have designed the menu based on the movie Never Been Kissed as a prank. Apparently, Drew Barrymore is a good friend of Edward Norton. (source)

22 On the DVD, there is a warning from Tyler Durden just after the Copyright warning.

Just after the DVD displays the Copyright warning, there is a fake warning from Tyler Durden. The warning just appears on the screen for a second. Read the image above for the exact words on the warning. (source)

23 Finally, in the last scene of the movie, a male genitalia pops up on the screen for a split second just like Tyler used to insert into films when he was working as a projectionist.

During the movie the Narrator lets the audience know that Tyler Durden inserts male genitalia projections in between movies when he was working as a projectionist. The movie ends with the exact same Tyler prank. (source)