404 - Unique 404 page not found
54 Languages, 1331 bytes

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Alumin,
Alice,
Whirl

This is getting very long, so at @MDXF's suggestion, I'm moving the subprograms and comments to a gist. The program here is only the final program. Thanks to MDXF again for providing the TIO test driver.

54. Whirl

##\$"404"#N#S# , ,,#$#?@\404!?@GJlICJlGCC*G^+0%=%=+0%x&amp;fqpqqqiipsoddddoiipsohphhhhyhhhh? &#9;  ?nnn4$4#!000110000011110000100000100000110000011001100000111100001110011000111000110000
#?\++++:----:++++:H@((((4O0O4O@((((()()()){}){x}x){}x()){}){}()){}){})&lt;[-]&gt;[-]-[&lt;+&gt;-----]&lt;+x%"404"?.⌂----.++++.&gt;11    &#32;
#i(N/"404"ooo@ENTHOEONHSSEONTHOEONSETssipceaiiiiiscejiiiijeeeejapmzaeeaeueewzaeeeaeewzaeeaeueewqs??|?)999|997+|++++!999777+++++!999997+++++!   &#9; &#32;
print(404)&#9;
 &#9;&#9;
 &#9;&#9;
 &#9;
__DATA__=1
# \"404"*CC'[-][ .-$_"404"&amp;]
"""pp
I want 404 of this
How much is it
#  -52, Z, -1                          #
#  -48, Y, -1                          #
#  -52, X, -1                          #
[-][
x::=~404
::=
x
]&lt;&lt;&lt;
&gt; 404
&gt;&gt; Output 1
red up two blue up red up one blue up red up two blue up
baaaaa bleeeeeeeeet bleeeeeeeeet baaaaa bleeeeeeeeet
a now 404 dollar off!
a has been selling out worldwide!
2 LEFT
2 LEFT
2 UP
2 LEFT
2 TEACH
1 LEFT
1 RIGHT
1 BOND
1 BOND
1 TEACH
2 YELL
1 FIGHT
2 YELL
1 TEACH
2 YELL
set ! 52
set ! 48
set ! 52
+------+
|$0011\|
|/000\0|
|0/0@11|
|00  10|
|0\10/0|
|\1100/|
+------+
$'main'
 \-444o000o444omm^

[-][$++++++++++++++++++++++++++++++++++++++++++++++++++++.----.++++.#
&lt;&lt;&lt;&lt;&lt;&lt;```&gt;&gt;&gt;&gt;.&gt;.&gt;.
]
ss"""
#7777777777777777777777777777724091557093543643404➡77▲▲▲²²▲²²¡▼▼▼▼¡▲▲▲▲¡⠎⡆⡎⡆⢐⠣⠃s&amp;&amp;&amp;&amp;~-&lt;&lt;:&lt;&lt;:&lt;&lt;&lt;:&lt;:&gt;&gt;&gt;&gt;&gt;&gt;&gt;&gt;:^||||G&lt;&gt;


Try it online!

Whirl is described as a Turning Tarpit [sic], which uses only two instructions, 1 and 0. 1 rotates the current wheel of instructions (either the math wheel or the operations wheel), 0 swaps the rotation of the wheel, and two 0s executes the instruction and switches wheels. The relevant instructions are on the first line:

000110000011110000100000100000110000011001100000111100001110011000111000110000

000110000 Switch to math wheel and execute math.not (math.val = 1)
011110000 Execute math.store, (memval = math.val = 1)
10000     Execute math.add, (math.val = math.val+memval = 1+1 = 2)
010000    Execute math.store (memval = math.val = 2)
0110000   Execute math.multiply, (math.val = math.val*memval = 2*2 = 4)
01100     Execute math.store (memval=math.val) and switch to the ops ring
110000    Execute ops.one (ops.val = 1)
011110000 Executes ops.intio, printing memval (4)
11100     Add one to memory pointer (memval=0)
1100      Execute maths.nop to switch back to ops ring
011100    Execute ops.intio, printing memval (0)
01100     Execute maths.store, (memval=maths.val=4)
00        Execute maths.intio, printing memval (4)


The leading 01s cancel each other out, and the trailing 01s cause a floating point exception.

Edit: fixed a bunch of broken stuff

(Commands to be careful about in the future: UDLR%"[.]o473psjw)

If any programs work for future languages or don't work for current or previous languages, please comment.