Possibly quit your job with a polyglot

Python 2 / Python 3 - 128

n=int(input())
s="Isf  tthhies ,b oIs sq ufiitn.d"
print(["not "*any([n%a<1for a in range(2,n)])+"prime",s[::2]+s[1::2]][1/2>0])

Identifies prime numbers with Python 2, get's you into trouble with Python 3.

Bonus

__help__ = """In the forloop I'm setting the values i to n.
To be a "notPrimes", it's n % i == 0 if: 1 < i, i < n. (We
do tests i<1/2 i==1, too). Then, prints resulting, prime text.
In case i==n: quit. Just if it aborts: try."""

# read integer from command line
n=int(input())

try:

    # primes = True
    notPrimes = False

    # try each i to n
    for i in range(n):

        # ignore 0 or 1
        if i < 1 / 2 or i == 1:
            continue

        # test divisibility
        if n % i == 0:
            notPrimes = True

    # print result
    if notPrimes:
        print("not prime")
    else:
        print("prime")

except:

    # if program aborts: print help and error code
    print(__help__ [::7])

Try it with Python 2 or Python 3! (In contrast to the golf version above the roles changed: Python 3 is the prime number identifier. Python 2 contains the Easter egg.)

Please, excuse my bad English in the help text! ;)

And I do use the word "quit". But somehow I need to describe when my program terminates. ;)


Bonus submission (C / C++11)

Primality testing using the usual naive method is so mainstream. That's why I have invented a brand-new randomized naive method! This test is as follows:

  1. Choose any integer d at random. It must not be smaller than 2 and larger than a bit more than sqrt(n).
  2. If d is a divisor of n, output not prime.
  3. If we made this test 20sqrt(n) times, output prime, else repeat.

If the number is composite, there is only very little probability (about 10-9) that it doesn't work. Of course, I don't believe the C/C++ pseudorandom number generator is powerful enough. That's why I use my own 256-bit LFSR generator!

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

/* A 256-bit linear feedback shift register generating pseudorandom
 * numbers (its period is 2^256 - 1).
 */
struct LFSRGenerator {
    unsigned seed[8];
};

void lfsr_init_generator(struct LFSRGenerator *gen){
    gen->seed[0] = 0xE840CC92; gen->seed[1] = 0xC440CAD0;
    gen->seed[2] = 0x40E6E6DE; gen->seed[3] = 0xC8DCD2CC;
    gen->seed[4] = 0xD0E840E6; gen->seed[5] = 0x4058E6D2;
    gen->seed[6] = 0xEAE24092; gen->seed[7] = 0x145CE8D2;
}
void lfsr_proceed(struct LFSRGenerator *gen){
    // LFSR taps are x^256, x^254, x^251 and x^246
    unsigned new_bit =
        ((gen->seed[7]>>0)^(gen->seed[7]>>2)^
         (gen->seed[7]>>5)^(gen->seed[7]>>10)) & 1;

    // shift seed right
    gen->seed[7] >>= 1;
    int cell;
    for(cell = 6; cell >= 0; cell--){
        gen->seed[cell+1] |= ((gen->seed[cell]&1)<<31);
        gen->seed[cell] >>= 1;
    }
    gen->seed[0] |= (new_bit<<31);  // put new bit
}
void lfsr_error(struct LFSRGenerator *gen){
    fprintf(stderr, "Error! Developer info:\n");

    int cell;
    for(cell = 0; cell < 8; cell++){
        unsigned val = gen->seed[cell];
        putc((char)(val&0xFF), stderr); val >>= 8;
        putc((char)(val&0xFF), stderr); val >>= 8;
        putc((char)(val&0xFF), stderr); val >>= 8;
        putc((char)(val&0xFF), stderr);
    }
    putc('\n', stderr);
    exit(1);
}
int lfsr_get_num(struct LFSRGenerator *gen, int min_val, int max_val){
    lfsr_proceed(gen);
    int mod_num = max_val-min_val+1;   // = number of possible results
    if(mod_num <= 0)
        lfsr_error(gen);

    // take 6 first cells and compute them mod 'modNum'
    unsigned long long result = 0;
    int cell;
    for(cell = 5; cell >= 0; cell--){
        result = ((result << 32) | gen->seed[cell]) % mod_num;
    }
    return (int)result + min_val;
}

/**********************************************************************/



void end_not_prime(){
    printf("not prime\n");
    exit(0);
}
void end_prime(){
    printf("prime\n");
    exit(0);
}



int main(){ 
    int number;
    struct LFSRGenerator gen;
    lfsr_init_generator(&gen);


    printf("Provide a number to check its primality: ");
    scanf("%d", &number);

    if(number <= 1){
        end_not_prime();
    }
    if(number == 2){
        end_prime();
    }

    // just to make sure:
    //  * make 20*sqrt(n) tests
    //  * generate random divisors from 2 to 111111/100000 * sqrt(n)
    //      (in case max range doesn't include sqrt(n)
    auto num_checks = (int)floor(sqrt(number)*20);
    auto max_range = sqrt(number);
    max_range /= 100000;
    max_range *= 111111;
    max_range = floor(max_range+0.5);

    while(num_checks--){
        int rnd_div = lfsr_get_num(&gen, 2, max_range);
        if(number % rnd_div == 0){
            end_not_prime();
        }
    }
    end_prime();
}

C++11 works correctly. However, C compiler seems to be outputting a faulty program for n > 2...

Note: remember that C needs -lm option (link math library) to compile successfully.

Look at max_range variable. C++11 keyword auto resolves to a "matching type" - in this case double. However, in C it's defined as a variable modifier (as static is) - it doesn't define the type. Thus max_range type is a default C type, that is int. When we "try" to multiply this variable by 1.11111, in C it gets "unintenionally" zeroed during division by 100000. We get an incorrect interval of random numbers to be generated and LFSR after proceeding its internal state generates an error, outputting the binary dump of the seed. That's "accidentally" The message If the boss finds this, I quit.\n

If you find the following faulty output:

Error! Developer info:
If the boss finds this, I quit.

incorrect, just remove the appropriate fprintf line.


Mathematica/Brainfuck, 260

If[PrimeQ[Input[]],"prime","not prime"](*++++++++++[>+++>++++>+++++++>++++++++++>+++++++++++<<<<<-]>>>+++.>++.<<<++.>>>>++++++.<++.---.<<<.>>>---.>-----.++++..<<<<.>>>++++.+++.>-----.<-----.>+++++.<<<<.>>>>+.<++++.+.>-.<<<++++.<.>>.<<.>>>>--.++++.<.>-.<<<++.*)

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Code Golf

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