Hello! I was wanting to see all of the Easter Eggs for myself, but I could not for the life of me find any sort of Easter Egg list online that wasn't in video format*, so here we are!
I am talking about the base story gameplay at the time of writing, and I hope to eventually include lists of Secrets in Endless Mode, and Karamari Hospital.

Please inform me of any misinformation or missed information!
I am very sure I've missed some!



*As stated in the description, I am keeping this as spoiler-free as possible so that these Easter Eggs can be experienced as freshly as possible. I found that watching a video for a list was redundant as I wanted to experience these for myself, not see them through a video.

• Distorted Teeth on the main menu screen instead of the usual logo.
  
• ‘Spooky's Sandcastle Of Land Sharks' on the main menu screen instead of the usual logo.
  
• 'Spooky's Homecooked BBQ’ on the main menu screen instead of the usual logo.
  (These are all random chance upon game start-up.)

• Room 165: There’s a poster of ‘Kyubey’ from ”Puella Magi Madoka Magica,” and a drawing of multiple characters, including ‘Hatsune Miku,’ on the classroom walls.
  
• Room 210: In one of the sub-rooms, there’s ‘Han Solo’ from “Star Wars” trapped in carbonite.

• Room 557: W.A.M. is seen when the record player stops and the player turns around. After W.A.M. disappears, the player can look back at the record player, and then turn around again to see him again.
  
• Room 554: When the player acquires the Axe, if they choose to swing it at Specimens and rack up 20+ points, they will achieve the Bad Ending after completing the Story. (You gain points by hitting Violent deer, Specimen 1, Specimen 2, Specimen 6, Specimen 11.)
  
• Room 710: Rule #3 in the playroom is thought to be a reference to the movie “Eegah.”
  
• Room 710: In the bottom right of the playroom shoe rack, there’s a skull in the cubby.
  
• Room 500+: The entry for Specimen 10 in the CAT-DOS will change to the 'true' entry after room 500.

• Room 810: There's a small chance a hanging figure will appear on the right hand side of the first room of the mansion-inside-a-mansion.
  
• Room 900+: Click 'MODIFY_HOUSE_LAYOUT' in the CAT-DOS machine six times, then open Specimen 9's entry, secret Specimen entries are shown.
  
• Room 900+: You can input the password ‘1235’ into the broken arcade machine to play a secret minigame. This password is found in the Brainframe Room, also after room 900.

• In any random room, you can encounter a repeating rectangular room called either "Howard's Room" or "PT Room." This room is a reference to the game PT.
  After completing two room 'loops' a note will appear on one of the tables.
  After 1-3 'loops' Howard will jumpscare you, although he does no damage and does not kill the player.
  After 3 'loops' you can exit the previously locked doors.
  
• You may come across a forked room, with a never-ending hallway in the middle. If you walk too far down this hallway, you will get stuck and Specimen 9 will kill you. This is unofficially regarded at Specimen 9's 'room.'