Q:

Filthy and Unique

In this question I will talk about programs as strings, this is strings of bytes, not characters.  How your resultant program is rendered or displayed is not important to this challenge, only how it appears in memory matters.
A pristine program is a program \$S\$ that when run does not error, however it will error whenever a continuous substring of size \$n\$, where \$1\leq n < \left|S\right|\$, is removed.
A filthy program is the opposite, it is a program \$S\$ that when run does error, however whenever a continuous substring of size \$n\$, where \$1\leq n < \left|S\right|\$, is removed, it does not error.
For this challenge an error is non-empty output to STDERR.
Your challenge is to write a filthy program that uses as many unique bytes as possible.  This means you will get one point for every unique byte that appears in your code with a larger score being better.  The maximum score is thus 256.

A:

Unary, 14 bytes
123456789ABCDE

This encodes the brainfuck program [, which errors due to unmatched brackets.
Removing bytes will result in >, <, +, -, ., , or the empty program, which are valid brainfuck programs.

A:

R, 3 bytes
qrt

Try it online!
A name of an object is a valid program in R. 
qrt is not the name of anything, so it returns an error.
q is the quit function
qr is the QR decomposition function
rt is the t distribution sampling function
qt is the t distribution inverse CDF
t is the transpose function

A:

Jelly, 4 5 6 7 8 10 11 bytes
“a”;⁽PFð+µU

Try it online!
Verify it.
Tries to add a string with an integer.
Some of the possible subprograms:
“a” is a string literal.
“a is the same string literal.
“ is the empty string.
“a”; concatenates "a" with itself.
...too many to enumerate them all.

