All five characters who get on the elevator have the color red in common: the Mechanic's satchel, the Old Woman's hair, the Young Woman's nails, the insignia on the Guard's uniform, and the Salesman's tie are all red.
The sign on the truck that the man lands on after he jumps off the skyscraper says "Bethel bread company", Bethel is considered to be a holy place, in the Bible it was one of the first places where the Hebrews met God.
There are the number "666" references: the building number is 333 (666/2); the elevator number is 6, and stops on floors 23 to 42 (2*3=6, 4+2=6); when the hockey match on TV turns on right after the elevator stops, the time of the match is 1:42, (4+2=6), the floor that Sara wants to go to is 42 (4+2=6), the floor that Ben needs to go to is 39 (9-3=6).
The trivia items below may give away important plot points.
Director M. Night Shyamalan admitted that Agatha Christie was an influence on the film, especially Christie's 1939 novel "And Then There Were None." They both share the same plot of a group of people with guilty pasts trapped in an isolated location, and die off one by one. The final plot twist is also the same. The villain is among them, thought to have been killed off earlier in the story.
Towards the end of the movie Detective Bowden tells the mechanic that it was his wife and child that was killed in the hit and run car accident on Bethlehem Pike. Bethlehem was the birthplace of Jesus.
After crashing his car and committing a hit-and-run, the Marine wrote "I'm sorry" on a car wash discount coupon. The coupon was for Trinity Service Station, and the Trinity represents the father, the son and the holy ghost. Seeing as the crash killed the detective's son and wife, it represents him being the father and they were the son and the holy ghost.