Can anybody explain me, where am I wrong?

Definitions:
ac, ab, bc - corresponding lengths
acb - angle between (ca) and (cb)

Such code:
...
acb = acos((ac*ac + bc*bc - ab*ab) / 2.0 / ac / bc);
..

gets AC.

Such code:
...
double spv(point x1, point x2, point y1, point y2){
    int i;
    double res = 0;
    for (i=1; i<=n; i++){
        res += (x2.coord[i] - x1.coord[i]) * (y2.coord[i] - y1.coord[i]);
    }
    return res;
}
...
acb = acos(spv(c,a,c,b)/ac/bc);...
...

gets WA25.

I had always thought that these two fragments must yield the same result. Why don't they?

me too. If you get WA27, use type double