Python, 35 bytes

Pretty simple.

print(a)########################### 

Prints out an undeclared variable then adds a comment to make sure that the error message (NameError: name 'a' is not defined) is shorter than the actual code. I could've probably done better.

Whitespace (wspace 0.3), 19 bytes

SSSSL SLS TSTS TLST LLL 

Error: w: divide by zero (18 bytes)

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The zero divisor error is the shortest error message produced by the wspace 0.3 reference interpreter (see the yspace project for a survey of other errors). Using mod in place of div would yield an equivalent result. The reference interpreter was written in Haskell, so arithmetic operations are evaluated lazily and it will not panic with divide by zero unless the illegal value is used by an eager instruction—printi in this case. To reach the minimum byte count, push 0 is written as SSSSL and end is appended.

push 0 dup div printi end 

Renaming wspace to w gives a shorter error message.

$ ln -s ~/bin/wspace w $ ./w codegolf/shortest_error.ws w: divide by zero $ ./w shortest_error.ws 2>| wc -c 18 $ wc -c < shortest_error.ws 19 

(This solution is part of the ws-challenges repo)

Raku (Perl 6) (rakudo), 38 bytes

die#iaj zav; xbsio la, n v km l s zioa 

When run using raku -e, this produces the 37-byte output

Died in block <unit> at -e line 1 

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YASEPL, 38 bytes

=p/0+000000000000000000000000000000000 

error message:

ERROR 8 divide by 0:( located at /0 

Go, 67 bytes

package main func main(){var f string = "aaaaaaaaaaaaaaaaaaaaaaaa"} 

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Gives the following error (66 bytes):

# command-line-arguments ../code.go:2:17: f declared and not used 

Bash, 14 bytes

2>&1 ~|cut -b1 

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This may or may not fit the rules, I'm not sure... If manipulating the error message is not allowed, I'll remove it. But I thought it was amusing enough to post.

It works by running ~ as a command to generate an error. But since 2>&1 redirects STDOUT to STDIN, the error is passed to cut -b1 which truncates the output to 1 byte.

C (Linux), 19 bytes (but code much simpler)

main ; 

Output:

Segmentation fault 

Emoji-gramming, 12 bytes

😊🎤📢 

Error, 69 bytes

💻 ⚠️ ➡️ 🚨 1 ⚠ 🎤 ↔️ 📢 😊 🎤 📢 None 

I don't know if the None and the trailing line feed in the output count or not.

The error means that the 🎤 (input variable) and the 📢 (output variable) are in the wrong place.

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Swift 5.9.2, 46 41 39 bytes

let heresARemarkablyLengthyIdentifier : 

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Error, 45 40 38 bytes (when compiled from standard input)

<stdin>:2:2: error: expected type : ^