Specification: Pad a Java String with spaces so that it is 16 characters long.
The wrong way to do it:
public String pad(String input)
{
   if (input == null)
   {
       for (int i = 0; i < 16; i++)
       {
           input += " ";
       }
   }
   else
   {
       for (int i = input.length(); i < 16; i++)
       {
           code += " ";
       }
   }

   return input;
}
The bug in the above code is that the += operator on a String will convert a null reference String to the 4 character String "null" so when input is null, the return will be a 20 character String starting with the word null followed by 16 spaces:
"null                "
A bug free implementation is:
public String pad(String input)
{
   StringBuilder builder = new StringBuilder();

   if (input != null)
   {
       builder.append(input);
   }

   for (int i = builder.length(); i < 16; i++)
   {
       builder.append(' ');
   }

   return builder.toString();
}