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Throughout the Jak and Daxter series, a fictional, translatable language appears in a variety of environments known as Precursor. Official guides often included legends that enabled players to translate signs in the open world Haven City and other instances of Precursor, often discovering easter eggs in the process.

For an unknown reason, exclusively to the 2012 PlayStation 3 port of Jak II released in the Jak and Daxter Collection, a particular texture on a barrel of Dark Eco appearing in the Drill Platform stage was altered; originally, in the PlayStation 2 version of the game, the barrel read "Barrel of Eco" in Precursor. In the PlayStation 3 version, the same Precursor text now instead reads "Barrel of Shit", and was possibly changed to mess with anyone diligent enough to translate Precursor in the middle of gameplay.
Contributed by Regen-33
There is a Precursor orb hidden out of bounds underneath the Underport stage that can be collected using a documented hovering glitch with the JET-Board found in both this game and Jak 3, or through the game's debug mode.
Contributed by BullZ'eye
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There is a glitch whenever Jak walks in front of a mirror that causes a slightly different model of Jak with dark grey horns to appear alongside the "reflected" second model of the room, initially suggesting that Jak was used as a test model for shaping Dark Jak's horns. However, a 2003 promotional contest advertisement hosted by Cartoon Network's programming block Toonami features a higher resolution model of Jak with white horns instead, suggesting that the model sent to Cartoon Network still had the horns on them and they just altered the model for the advertisement, or that Jak was originally supposed to have visible horns like Dark Jak.
Contributed by BullZ'eye
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Although not referenced in game, Pecker is a Monkaw (half monkey and half macaw).
Contributed by BullZ'eye
During development, early ideas for new eco powers revolved around time manipulation. Although unused in the final version of the game, these eco powers are present in a preview build - Jak would've been able to manipulate time by accelerating it (increasing his movement and firing speed), decelerating it (slowing enemies down), or stopping it altogether.
Contributed by BullZ'eye
One mission went completely unused in the final build of the game, having Jak and Torn protect the city wall from a Metal Head invasion. The fight would end with Torn revealing his tatoos symbolize the guards' loyalty to the city.
The mission is present in a preview build of the game, complete with character dialogue and ending cutscene.
Contributed by BullZ'eye
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Design documents show that the relation between Jak and Mar was to be explored in more depth before the end of the game. In early plot revisions, Jak and Sig would discover Mar was a time traveller who vanished after founding the city.
Mar's time travelling device would've been found in the Underport.
Contributed by BullZ'eye
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The earliest iteration of Dark Jak would've made him faster, completely invincible and able to use items that would otherwise hurt Jak.
This would come with the added drawback of always ending the transformation with one hit point left.
Contributed by BullZ'eye
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Each civilian vehicle has its own name: One-seater vehicles are Zoomers, two-seaters are Flyers and three-seaters are Cruisers.
Contributed by BullZ'eye
"Construction Site" is a relatively unimportant level in the game, with only a cutscene being played there and little else for the player to do. It is very likely, however, that this level was at one point meant to have a bigger role in the game. The level has a lot of collision at places that are not normally reachable, and there are text entries for unused mission names, including "Climb Construction Site" and "Find Baron at Construction Site"
Contributed by BullZ'eye
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There is a picture of the Naughty Dog developers hidden out of bounds inside a building in Haven City.
Contributed by BullZ'eye
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Near the stadium is a billboard of Ratchet and Clank from their appearances in Ratchet & Clank: Going Commando.
Contributed by ReakMayhem
Sandover Village can be accessed and explored though a glitch in the first cutscene. However, since the area was only meant to be used in said cutscene, the village does not have any characters and the levels Forbidden Jungle, Geyser Rock, Misty Island, Sentinel Beach, and Fire Canyon are inaccessible as there is nothing to call up for these areas.
Contributed by gamemaster1991