Many times I have stuck something in my system with the preconceived notion that is would be better. It just had to be better. But it was not. I'd spend hours trying to convince myself that is was really better, but something else was off. In the end it was not better. If I am just imagining things, it would have always sounded better in those situations, since I was favorably biased going in.
In a classical music context, there are many who think the conductor does not matter. After all, the top orchestras have world class musicians in them who know what they are doing.
About six-seven years ago I heard a young (roughly 32 year old) unknown guest conduct in Baltimore. I was very impressed. Six months later I heard him conduct again. I heard the same excellence. Was I imagining things? Well, he was just hired (still under 40 years old) to be the new Music Director of the New York Philharmonic. The only other time they hired someone young was Bernstein. People that young just don't get jobs like that with million dollar salaries. So I can't be the only person who heard what I heard.