I probably should have noticed this long ago, but I've always used the same optimization setting (maximize speed). Just now I tried turning off vectorization (/Qvec-) and saw that my program generates different results. I then ran through a few different optimization settings, focussing on a single number generated by the program. These are the values I got:
/Od /O1 /O2 /O3
2365 727 727 727 /Qvec-
2258 2258 /Qvec
Without vectorization, any optimization produces a change over no optimization. With vectorization, the results for /O2 and /O3 are the same but different from those with no vectorization.
I'm using 11.0.072. Have I just discovered the reason why this version is no longer available, and why Wendy Doerner was helping me obtain 11.0.075 (which I still haven't managed to obtain)? If so, my interest in getting this fixed version has just jumped by an order of magnitude.