The rate of fire of weapons of the time periods of the missions which correspond to the events in the first Killzone is significantly faster than the rate of fire of the weapons in that game.
The front sight of the ISA's main rifle cannot be seen when aiming through the holographic back sight; instead there is only a flat top with no sight.
Mandor Savic's picture is reversed in the briefing, with his breathing tube on his right-hand side. His breathing tube is shown to be on his left-hand side in every other occurrence.
The opening cutscene shows Helghast fighting during the original rebellion which resulted in them being banished to Helghan. Since they were humans before the conditions of Helghan changed their biology into Helghast, they would have been human - not Helghast - when they were fighting in the rebellion.
In the game the Player is given the option of destroying security cameras in order to avoid detection. This tactic would in a realistic scenario be folly, as destroying a camera would alert the operator to something being amiss.
The Intruder dropships have an open troop compartment, carrying troops on the outside. This is a very flawed design, as it would leave the troops within exposed to both enemy fire and harsh atmospheric conditions as well as imperil them to falling out of the troop compartment during evasive manoeuvres or high speed drops.