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In the mission Amendment XXV, in the wing in which Morris initially can be found, is a hall with a fireplace that isn't lit. Throwing a lady's briefcase (from the lady that goes for a smoke at the beginning of the mission) into the fire three times correctly, causes the marines to all form a line and start dancing the can-can. A marine suit is also needed for this.
Contributed by EFB01
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A Hitman website is visible on a computer screen near the start of the mission "A Dance with the Devil".
Contributed by EFB01
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In the mission "Till Death Do Us Part", there is a small coin near the water at the front of the mansion. If the player shoots it, the group of guests which were previously fighting each other will lose their clothes and run towards Agent 47, clapping their hands. After some time, they will resume fighting, though they will still only be wearing underwear.
Contributed by EFB01
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In the level "Dance With the Devil" is a kitchen crate in the freezer room of the Hell party. Its description reads, "Allan please add details", presumably accidentally left in by game developer Allan Hansen.
Contributed by DeerBoarDude