You are missing a break
case '\'': {
if (wskaznikStosu < 1) {
printf ("\nZa dużo operandów \n");
return 1;}
else {
int a = stos [wskaznikStosu - 1];
wskaznikStosu --;
stos [wskaznikStosu] = not (a);
wskaznikStosu ++;
}
}
// YOU ARE MISSING A BREAK HERE AND FALLING INTO THE DEFAULT
default: {