This is where it gets confusing, isn't it?
It depends on an individual person's perception. Let's take a look at two things pointed out from Halo.Bungie.Org's story section from Fall of Reach:
1. Pillar of Autumn uses rotating sections to simulate gravity (FoR p.274) yet no such mechanism is seen in-game
2. UNSC frigate Allegiance is referred to as the Alliance, and then shortly thereafter as the Allegiance again (FoR p.145, 154)
Inconsistency #1 is an example of a retcon. This is a theory that Nylund put together to explain why there is gravity on human ships. Later fiction sources confirm that the human ships have a primitive form of artificial gravity instead.
Inconsistency #2 is an example of an error. More than likely, Nylund was up late at night and simply made a typo.
That wasn't too confusing.
Yes, but then you get to the more confusing “gray areas”. This is where it can get confusing:
The Battle Rifle was a prototype in Contact Harvest and a new weapon in First Strike. However, it is used in a normal military situation in Cole Protocol.
Retcon or error? As of now, as far as the big picture is concerned, it could go either way. However, one note of interest is the fact that Halo Wars (which takes place only a few years before Cole Protocol) originally had some Marines use the Battle Rifle, but they were removed for canon's sake.
This is going to be really nerdy, isn't it?
Let's examine some more:
Spartans did not receive shields until 2552 with the Mark V MJOLNIR armor. However, the Spartans in Halo Wars all have energy shielding
This has been cited by the developers as a sacrifice for the gameplay of Halo Wars. In fact, they reported to the forum members that it was actually just a representation of regenerating stamina with the shield sound effects solely for familiarity. Despite this, the Halo Wars instruction manual states that they have energy shielding. Retcon or error?
According to Ghosts of Onyx, Onyx was deemed classified at the beginning of the 26th century. However, Professor Anders mentions it at a lecture in Halo Wars: Genesis, over 25 years later
This is another example of Nylund screwing up his own fiction. Retcon or error?
According to the Bungie.net timeline and the Halo Encyclopedia, the Battles in New Mombasa in Halo 2 both took place on October 20, 2552. However, Halo 3: ODST's script and promotional material shows it place on October 23rd, 2552.
Retcon or error?
According to the Halo Wars strategy guide, the Prophet of Regret arrived at the Apex Site prior to the events of the game. However, Waypoint's timeline says that he didn't arrive until after the battle of Arcadia.
That particular section of the strategy guide never made much sense to me. If Regret was already there, why would he need Arcadia and Harvest? Still, it's debatable. Retcon or error?
According to the Halo Wars guide, Regret arrived at the Apex Site prior to the events of the game. However, Waypoint's timeline says that he didn't arrive until after Arcadia.
Retcon or error?
If human communication technology is not able to navigate slipspace without a special probe, how was Keyes able to communicate instantaneously with Stanforth, far away, before Sigma Octanus.
It's highly possible that the UNSC has methods to communicate instantaneously from system to system while the probe is just special because it can go anywhere. However, given the little information available, this is debatable. Retcon or error?