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In chapter 3, it's possible to find a teleporter that will guide you to a room with a cake in it. On the wall is the word "surprise!" written multiple times. Finding this room will unlock the achievement "The Cake". This is a reference to Portal.
Contributed by Funland47
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In chapter 2 is a hidden cave. Within the cave is a sword sticking out of the ground next to a skeleton with an exclamation mark above its head. Finding this will unlock the achievement "WoW!" This is a reference to World of Warcraft.
Contributed by Funland47
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Soon after crashing the 4x4 in chapter 1, it's possible to find a hatch hidden in the ground. Standing over the hatch for a short period of time will unlock the achievement "Found!"

This is a direct reference to the TV series "Lost".
Contributed by Funland47