Did you know that in Halo 4 all the weapons have clips in reloading? You can see this really clear while using the Rail gun, the clip while reloading has a round in it.
In Halo reach a double beatdown has an alternative animation, this is clearly seen while killing an elite instead of the standard melee animation, your spartan hits the elite in its stomach (when using an AR)
Halo CE had a different lore to begin with than the lore we all know and love while adjusted in the other Halo's it was said the Masterchief was the last of his kind and actually a Cyborg. While in the release they changed this, it is still the lore on the blister packages of the action figures.
In Halo reach most weapons do not fire bullets from the right perspective, to compensate for the hitboxes and animations, most weapons fire bullets in third person, somewhat upward and away from the gun.
Also the lore around the flood is inconsistent all over the games, it is said they can only take over living creatures, while in Halo 3 it is shown they also take over dead bodies. The reason of this is likely for gameplay reasons. There is a witness stating that by playing dead the flood ignored him completely.
Most weapons have different versions. Like the assault rifle in Halo CE has 60 bullets, while only 32 in halo 3 to 4. The change was made for the improving of gameplay. Bungie used the lore to explain this. For instance the AR in Reach has a completely different look as if its unplated. The reason for this is that it's a prototype weapon.
On the cover of Halo CE the chief is holding a fuel rod cannon, but this weapon is only accesible in Multiplayer.
The maps Ragnarok and Valhalla are actually reïmaginings of the map Bloodgulch.
The Masterchief was not the only spartan left during the covenant war, but as most of the marines considered him a legend, they used this ruse to increase morale, during the battle of New Mombasa in Halo 2, there are other spartans fighting. You can see in Halo 3 that the marines respond to him in awe as he walks out of the pelican.
Miranda Keyes her mother is Doctor Catherine Halsey.
There is a soundlog in Halo 4 after the forward unto dawn crashed, which stated that since Cortana was created with organic material, she would likely not have rampancy, because the rules of normal AI did not apply to her.
Since Cortana was created with Halsey's DNA she shares a lot of similarity with Halsey and the two of them did not see eye to eye on most ocassions.
Both Spartans Cortana picked out, Noble Six and John-117, are the only two spartans to ever recieve the classification of extremely lethal.
Every Halo (besides wars) has the protagonist falling out of orbit
Halo CE with the escape pod.
Halo 2 with Chief returning the Covenants present.
Halo 3 in the beginning.
Halo Reach with Jorge dropping Noble Six
Halo 4 with Chief plummeting towards requiem
Halo ODST when you drop.
The funny thing is spartan armors are not designed to handle that kind of drop. In the games with the Masterchief this is considered to have to do with his "luck". In Halo Reach, Noble Six was wearing a pack with some emergency supplies.
The Didact and the Librarian were part of the Halo lore since Halo 3, their story is told in the terminals scattered around the game.
The reason that all Forerunner weapons disolve their victim and most of their "machines" disolve, has to do with the flood. No living tissues means no host and no machinery means nothing to operate. While there are a only forerunner AI in (promethean knights etc.) remember that during the war with the flood, a lot of Forerunners got infected as well, allowing the flood to operate forerunner tech. The reason the Prometheans disolve is has to do with relocating their AI, but also to render them not being able to be directly used if the Didact gets infected.