In the final version of the game the "Galaxy" music from the soundtrack sounds different than its beta counterpart. While the composition is the same the final version is much more subdued.
During mission 6, there are two Marines standing near a forklift talking about loading crates. The two Marines are voiced by Conan O'Brien and Andy Richter, two American late night TV talk show hosts.
There is a warning label on side of the M6 Spartan Laser that reads "Big Scary Laser. Do not look into beam with remaining eye".
An unused weapon can be found in the game's files named 'Burst Pistol'.
After Halo 3 was finished, Bungie considered developing Halo 4, but they decided to develop Halo: Reach instead.
An unused model of Halo Reach's Scarab can be found in Spartan Ops Map "Harvester" data files. It wasn't meant to spawn any where in the map at any point.
At the end of the credits, there is a list of names under the title "Heroes Never Die", paying tribute to those who died during the game's development. One of these names is Cade Ainsworth, a 16 year old Halo fan who died of cancer. 343 Industries heard of his passing and placed his name in the game to honor him.
If you aim a hologram at a dead body, there is a chance that the hologram will teabag it.