Double-duty quine: quine!dlroW ,olleH
JavaScript (ES6), 42 38 bytes

f=_=&gt;/\//g&amp;&amp;"f="+f||"!dlroW ,olleH"&gt;=_


Reversed

_=&gt;"Hello, World!"||f+"=f"&amp;&amp;g//\/&gt;=_=f


Explanation

When reversed, it becomes an anonymous function that returns the string Hello, World!.

The regex /\//g becomes a comment when it is reversed, which allows the syntactically invalid &gt;=_=f to be commented out in the reversed code.