Videos
Tony Hawk's Pro Skater - Did You Know Gaming? Feat. Brutalmoose
 
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In Tony Hawk's Underground 2: Remix, on the Santa Cruz stage exclusive to that version, if you travel out-of-bounds behind a wall near the Amusement Park/Arcade area, you can find a hidden picture of a distorted red face.
Contributed by MehDeletingLater
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Within the debug menu of T.H.U.G 2, there is an option to instantly grant the player a score of five million points.

When this debug function is activated, the text near the bottom of the screen in which the name of a trick performed during a combo would appear, instead the phrase, "You cheat like Kurt" fills in.

References to "Kurt" are a running inside joke within Neversoft, with numerous respective instances occurring in both Tony Hawk's Underground 1 as well as Underground 2 as Easter Eggs.
Contributed by Regen-33
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Accessing the arcade cabinet used for a specific goal on the Skatetopia level reveals a hidden message written out in the high score initials.

The message, reading, "TEN HARDCORE YEARS KICKING YOUR ASS", which is a reference to Neversoft Studios developing the Tony Hawk game franchise for ten years.
Contributed by Regen-33
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Kenny the Koala, the mascot from the zoo level in Tony Hawk's Pro Skater 4, appears as a statue in the Australia level of the game.
Contributed by DeerBoarDude
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The internet icon "Star Wars Kid" appears as an easter egg Spoiler:in Boston. At first, he just sits watching the television in front of him, but on command he does his signature "moves".
Contributed by mrlenny94