1 Sans actually exists outside of the Genocide Run
We all know that, but most people pretend not to. - Garythesnail
he does - Kryptical
2 Alphys is by far the darkest character in the game
Oh...come to think about it Alphys's character is pretty dark. Didn't it hint that she committed suicide in the genocide run?
The True Lab was like a horror game!
She is crazy, she made these mutant creatures by injecting determination into them. Her own girlfriend, Undyne, ends up melting because of too much determination. Worst of all, Alphys turned Snowdrake's mother into one of those. - Canuck101
3 All of Gaster's messages can be translated into English
4 Sans is white
WHITE, WHITE, WHITE - xandermartin98
5 Alphys was literally portrayed as a female version of Ren Hoek in the fanfic "Alphys Hoek", which parodied no less than six of the greatest episodes from Ren & Stimpy
6 Alphys X Amalgamates actually became a real, full-on sexual shipping at one point, and I have sincerely regretted being responsible for its creation ever since
7 It got overrated because it actually was a legitimately great game as well as a revolutionary work of art
8 Its creator actually spent the full 60-80 hours playing Xenoblade Chronicles prior to Undertale's creation, deciding to make it much shorter than XC as a result
9 Nintendo has eagerly suggested that Toby Fox should make an agreement with them to get Undertale ported onto the Wii U and 3DS
This needs to happen... (please Toby? ) - Antifi
Really?! That'd be so cool!
10 Burgerpants' official in-game sprites are just about literally identical to his original concept art
11 Alphys is also the hottest character in the game
12 You can actually get into Sans' room
13 The relationship between Sans and Papyrus is staggeringly similar to that of Mario and Luigi
14 The game's title literally has the exact same number of letters as the word "overrated"
Don't say Gosh's name in vain, me!
Oh my gosh you're right
15 All of the Amalgamates are basically just evil clones of previously encountered enemies (ie Endogeny to Greater Dog)
16 ffSade's "UNDERTALE - amalgamates" fanart is actually a background painting of the Amalgamates merged with a foreground drawing of Alphys
17 Its creator also made the infamous Earthbound Halloween Hack featuring Dr. Andonuts as the main villain
18 So Sorry is based off of the fetish fursona of a spoiled-sweet furry who ended up donating a positively ridiculous amount of money into the game's Kickstarter funding
19 The game ended up accumulating ten times the amount of Kickstarter funding that was actually needed in order to create it
20 The Memoryheads are most likely fragments of Gaster
21 Papyrus' suit has the insignia crest of Starmen from Earthbound
22 Muffet was literally funded into the game
23 Alphys' giant bag of dog food was actually intended for Endogeny
24 There has actually been art involving Alphys getting her feet worshipped by Endogeny
25 Endogeny licks Alphys' feet in pretty much literally every Alphamalg fanfic to date so far
26 The ghost living inside Mettaton's robotic body is actually female
27 Mettaton looks suspiciously a lot like David Bowie
28 Sans isn't Ness
29 Undertale marks the second time that Megalovania has been reused officially by Toby Fox
30 Over 70% of the entire soundtrack is comprised of leitmotif
31 Sans and Papyrus are often viewed as sex symbols by the fandom, even though they are both skeletons
32 The Annoying Dog is actually Toby Fox's in-game portrayal of himself
33 Temmie's eyes, nose and mouth are all detachable from her face
34 Napstablook gives you negative experience points if you choose to "kill" him
35 The Amalgamates are literally invulnerable to attack
36 Lemon Bread has 9999 DEF
37 The real reason why Lemon Bread says "welcome to my special hell" is because it is a fusion of the shy Moldbygg, the even shyer Shyren, and the terrifyingly clingy Aaron
38 The song "So Cold" from the soundtrack is randomly generated
39 The song "Another Medium" from the soundtrack uses the drumming soundfont from Cave Story
40 The Dreemurrs are based directly off of the Mimigas from Cave Story and are equally as adorable
41 The songs "Here We Are" and "Amalgam" from the soundtrack both use the soundfont from Earthbound
42 The song "Amalgam" from the soundtrack literally has Giygas' breathing sound effects from Earthbound in it
43 The character that gets his head eaten by butterflies in Reaper Bird's attacks is actually named "Everyman"
44 Omega Flowey's design was literally tossed together in Photoshop; in fact, its true name is actually Photoshop Flowey
45 There are quite a few character parallels between Undertale and Super Paper Mario
46 Alphys' bathroom is actually an elevator
47 There has been art of all of the different alternate-universe portrayals of Sans having an orgy with each other
And it's great.
48 The Amalgamates in Underfell look exactly the same as the original ones
49 Endogeny got turned into a quilt once, in a rather bizarre AU called "Pillowtale"
50 Underfell Sans has a gold tooth