The following four little pieces should give identical 
results:

$gcc test.c; ./a.out
-2, -2, -2 , -2, 0, -2
-2, -2, -2 , -2, 0, -2
-2, -2, -2 , -2, 0, -2
-2, -2, -2 , -2, 0, -2


but they dont:

gcc test.c; ./a.out
-2, -2, -2 , -2, 0, -2
-4, -2, -2 , -2, 0, -2
-2, -2, -2 , -2, 0, -2
-2, -2, -2 , -2, 0, -2

$cat test.c
void
main(void) {
        int  a, b, c, x, y, z, A, B, C, X, Y, Z;

        a=-2; b=-2; c=-2; x=-2; y=-2; z=-2; 

          (y -= b);
          (c ? a : b) += (x ? y : z);

        printf("%d, %d, %d , %d, %d, %d\n", a, b, c, x, y, z);

/*------------------------------------------------------------------*/

        a=-2; b=-2; c=-2; x=-2; y=-2; z=-2;

        (
          (y -= b) ?
          (c ? a : b) :
          (c ? a : b)   ) += (x ? y : z);

        printf("%d, %d, %d , %d, %d, %d\n", a, b, c, x, y, z);

/*------------------------------------------------------------------*/

        a=-2; b=-2; c=-2; x=-2; y=-2; z=-2;

        (
          (y -= b) ?
          ((c ? a : b) += (x ? y : z)) :
          ((c ? a : b) += (x ? y : z))   );

        printf("%d, %d, %d , %d, %d, %d\n", a, b, c, x, y, z);

/*------------------------------------------------------------------*/

        a=-2; b=-2; c=-2; x=-2; y=-2; z=-2;

        (
          (y -= b) ?
          ((c ? (a += (x ? y : z)) : (b += (x ? y : z)))) :
          ((c ? (a += (x ? y : z)) : (b += (x ? y : z))))   );

        printf("%d, %d, %d , %d, %d, %d\n", a, b, c, x, y, z);



}

Release:
2.95.3 20010315 (SuSE), 3.1

Environment:
Reading specs from /usr/lib/gcc-lib/i486-suse-linux/2.95.3/specs
gcc version 2.95.3 20010315 (SuSE)

or

Reading specs from /pkg/gnu/lib/gcc-lib/sparc-sun-solaris2.8/3.1/specs
Configured with: ../configure --prefix=/pkg/gnu --enable-languages=c++,f77
Thread model: posix
gcc version 3.1

How-To-Repeat:
just compile and run:

$gcc test.c; ./a.out