When have A/B comparisons led you astray?

Placebo effect. My brother went to audio store to buy Cabasse speakers. Salesmen presented first speaker that sounded very good. Then he played a little more expensive speaker that sounded much better for a little more and finally even more expensive speaker that sounded incredible but my brother could not afford it. He then left my brother in silence for a while taking care of another customer and came back with another option - brand new Cabasse design that plays almost as good as the latest most expensive one, but cost much less. After listening - my brother said that this is a great bargain, sounding almost as good as the most expensive one, an bought it. At the end salesman told him that it was the same, least expensive speaker he listen to first.

"i just change stuff. period, and don't worry about it"

- So do I. I found that switching A/B is not as good as playing one component for a week with different types of music then do the same with the other. Constant switching between components to see what improved and what not has less value to me than general impression after week of listening.