Deletion of a blank line in source code which causes unexpected functionality
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.