Find the largest prime under a number... but only for the test cases The Code

Your program should take an input of n > 2 and output the largest prime less than or equal to n. You can choose how the number is input (standard input, read from a file, etc). For example:

Input  Output
3      3
9      7
28     23
486    479


Simple enough. And if a program passes all of the test cases it's gonna work fine, right?

The Underhanded

Too bad your program will only pass the test cases. It should work properly for 3, 9, 28, and 486 and no other numbers. Since this is an underhanded challenge, this shouldn't be obvious to a person glancing over the code. 

Bonus points if the program still outputs something and if the output isn't obviously composite (getting 555 for 556 would be a little suspicious). This is a popularity contest, so the most popular answer wins. Which means the bonus points are completely meaningless. Oh well!
C
Results will be correct for the test cases, all others will be wrong for one or more reasons. The key is using the specifier %g which will round enough for the test cases but show the others as real numbers. Above inputs of 614, the results are all less than zero. Assuming inputs are in the integer domain!
#include &lt;stdio.h&gt;
#include &lt;stdlib.h&gt;

int main(int argc, char **argv)
{
    // Convert input
    double x = atof(argv[1]);

    // Cubic prime determination function
    double y = -1.386269645E-5 * x * x * x + 7.572051723E-3 * x * x + 5.774239804E-1 * x + 1.199953896;

    // Print output, use %g because it guarantees the shortest output
    printf(&quot;First prime less than or equal to %g is %g - happy day.\n&quot;, x, y);

    // Done!
    return 0;
}

Sample output:
$ ./fake 2
First prime less than or equal to 2 is 2.38498 - happy day.
$ ./fake 3
First prime less than or equal to 3 is 3 - happy day.
$ ./fake 4
First prime less than or equal to 4 is 3.62992 - happy day.
$ ./fake 5
First prime less than or equal to 5 is 4.27464 - happy day.
$ ./fake 6
First prime less than or equal to 6 is 4.9341 - happy day.
$ ./fake 7
First prime less than or equal to 7 is 5.6082 - happy day.
$ ./fake 8
First prime less than or equal to 8 is 6.29686 - happy day.
$ ./fake 9
First prime less than or equal to 9 is 7 - happy day.
$ ./fake 28
First prime less than or equal to 28 is 23 - happy day.
$ ./fake 486
First prime less than or equal to 486 is 479 - happy day.