If a==1:

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If a==1: is esoteric programming language, where program ends correctly, if accumulator==1, but if accumulator!=1, program outputs f"ERR: {a}".

Commands

command	description
+	inc
-	dec
.	out

Dont confuse commands with BittyLang

Interpreter in python

a=0
for c in input():
    if c==".":
        print(a)
    if c=="+":
        a+=1
    if c=="-":
        a-=1
if a==1:
    break
if a!=1
    print("ERR:", a)
    break

Stretching the rules a bit, you could interpret the output as ASCII, in which a Hello World program is possible.

Examples

XKCD Random Number

++++.

XKCD Random Number without ERR

++++.---

Hello world program without ERR (line breaks added for readability, they are NOP.)

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++.
+++++++++++++++++++++++++++++.
+++++++.
.
+++.
-------------------------------------------------------------------.
------------.
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++.
--------.
+++.
------.
--------.
-------------------------------------------------------------------.
--------------------------------