Deletion of a blank line in source code which causes unexpected functionality Write a script that works fine when a blank line is present in the flow of program logic, but breaks or causes unexpected behaviour when that line is removed.

Avoid standard loopholes and stupid answers. Your program should do something "useful," even if all it does is add a few integers.

Removing an empty line from a multiline string doesn't count, unless the expected output from that multiline string shouldn't technically change (ie: if you're removing all blank lines matching ^$).

Whitespace foolery isn't allowed either; don't do weird things with carriage returns, tabs, spaces, etc.
Javascript

Opens an alert popup with "Hello, World!" if the blank line exists, otherwise alerts "0".

(function(){
    alert(~(arguments.callee+[]).indexOf('\n\n') &amp;&amp; "Hello, World!");

})();


Demo

Explanation:

arguments.callee is a reference to the self-executing anonymous function.

Appending a blank array ([]) casts the function to a string, which gives you its source code.

indexOf returns -1 if the string is not found. 

Performing a bitwise not operation (~) results in -1 being turned into 0, which is a falsey value.