static int h263_get_modb(GetBitContext *gb, int pb_frame, int *cbpb)
{
    int c, mv = 1;

    if (pb_frame < 3) { // h.263 Annex G and i263 PB-frame
        c = get_bits1(gb);
        if (pb_frame == 2 && c)
            mv = !get_bits1(gb);
    } else { // h.263 Annex M improved PB-frame
        mv = get_unary(gb, 0, 4) + 1;
        c = mv & 1;
        mv = !!(mv & 2);
    }
    if(c)
        *cbpb = get_bits(gb, 6);
    return mv;
}

This weakness involves numeric computation errors, such as integer overflows, underflows, or precision losses, which can lead to miscalculations and exploitable behaviors in software.