Write a "Hello" interpreter
Lenguage, \$1.42 \times 10^{122}\$ bytes
minus a lot of bytes thanks to Kevin Cruijssen and Bubbler
hhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh...

This is 142099843608359281286315447494338058415442968773543757980908246691462388164856076679905341690709953072132211450166077106439 hs, which also makes it a valid Hello program, though not one you'd want to run. The original brainfuck code is 140 135 bytes:
,[&gt;-[&lt;--&gt;-----]+&lt;--[&gt;]&gt;[[--&gt;-[&gt;&gt;+&gt;-----&lt;&lt;]&lt;--&lt;---]&gt;-.&gt;&gt;&gt;+.&gt;&gt;..+++.&gt;&gt;.&gt;.&lt;&lt;&lt;.+++.------.&lt;&lt;-.[&gt;]&gt;&gt;&gt;&gt;,&gt;]&lt;]&lt;[[&gt;++&lt;+++++]&gt;-.+++++++++++++..&gt;]

Try it online!
This prints Hello World every time it sees a h through a modification of the shortest known Hello, World! program, stopping the loop and printing err if it sees anything other than a h.