Find an Illegal String The challenge is to find a string of characters that cannot appear in any legal program in your programming language of choice. That includes comments, strings, or other "non-executable" parts.

Challenge


Your program may be specific to a particular version or implementation of your language's compiler/interpreter/runtime environment. If so, please specify the particulars.
Only standard compiler/interpreter/runtime options are permitted. You cannot pass some weird flag to your compiler to get a specific result (e.g. passing a flag to convert warnings into errors).
If your programming language requires a specific encoding (e.g. UTF-8), your string must also be correctly encoded (i.e. strings which fail solely due to character decoding errors are not allowed).
Every individual character in your submission must be admissible in a legal program; that is, you can't just use a character which is always rejected.
The compiler/interpreter/runtime must give an error when given any source code that contains your string as a substring. The error does not have to be the same across programs - one embedding of your string might cause a syntax error, while another might cause a runtime error.


Scoring


Shortest illegal string for each language wins.
You should explain why your string is illegal (why it cannot appear anywhere in a legal program).
Dispute incorrect solutions in the comments. More specifically, you should provide a link to TIO or equivalent demonstrating a legal program (i.e. one that doesn't produce any errors) that contains the proposed substring.
Some languages (e.g. Bash, Batch, Perl) allow arbitrary binary data to be appended to a program without affecting validity (e.g. using __DATA__ in Perl). For such languages, you may submit a solution that can appear only in such a trailing section. Make sure to make a note of that in your answer. (The definition of this "trailing section" is language-dependent, but generally means any text after the parser has entirely stopped reading the script).


Example

In Python, I might submit

x
"""
'''


but this can be embedded into the larger program

"""
x
"""
'''
y
'''


so it isn't admissible.
Changeling, 2 bytes






That's two linefeeds. Valid Changeling must always form a perfect square of printable ASCII characters, so it cannot contain two linefeeds in a row.

The error is always a parser error and always the same:

This shape is unpleasant.


accompanied by exit code 1.

Try it online!