A list of secrets and references in Downfall. Note that some have corresponding achievements.
I Have No Mouth, and I Must Scream Edit
In the garden of Quiet Haven Hotel, a message can be seen inscribed on one of the pillars of the garden fence, which cannot be interacted with. This message looks similar to AM's "Message of Hate" seen inscribed on a giant, monolithic pillar in the intro of the adventure game I Have No Mouth, and I Must Scream.
Lucky Dip Edit
In the "Lucky Dip" option on the extras menu, it's possible to find some concept art for the original 2009 version of Downfall. Finding this will earn the player an achievement.
The Slenderman Edit
The Slenderman can make a surprise appearance when you're playing as Agnes in the forest, if you go back after entering a building similar to the one from the famous free indie horror game Slender: The Eight Pages. Remigiusz Michalski has shown a fascination for the game and The Slenderman mythos before, with The Slenderman also having an appearance at the end of the the anniversary trailer for The Cat Lady. The player is awarded an achievement for this.
Noises She Makes In Their Bed Edit
Joe Davis plays the song used for the main theme of The Cat Lady, the previous game in the series that Downfall is in. This reference also works in the fact that the person who wrote and played the song, Jesse "Warmer" Gunn, is the voice actor of Joe Davis, being a fan of Remigiusz Michalski's games since the original 2009 version of Downfall. Another reference also appears in the same scene, with Agnes saying the song sounds like Coldplay, a callback to a line in The Cat Lady. An achievement named after the song is awarded to the player for this.
When Agnes wakes up in a random hotel room, she can turn on the radio. Doing so will play a song by Jesse "Warmer" Gunn, one of the outsourced musicians for the game. The name of the achievement for doing so is a reference to the band Radiohead, with the image for the achievement appropriately being Agnes with a radio for a head.
If Agnes lies that she'll drink the soup, The Queen of Maggots will say her true name is Lorelai. While it's unknown if The Queen of Maggots is being truthful, this is a reference to the game of the same name, Lorelai, which is said will be the last game of the trilogy Downfall is a part of. An achievement is awarded for doing this, the image of which is the woman that's been seen in most promotional material for Lorelai.
Just Do It Edit
Choosing certain dialogue options when talking with Dr. Frank Zellman near the end of the game will cause him to reference the famous 2015 Shia LaBeouf meme "Just Do It." This is referenced further in the image for the achievement, being Joe doing the Shia LaBeouf pose in front of a green screen.