Did you ever mismatch equipment ?

Sometimes we regret having made a certain purchase, not always does it bring us what we expected. We know that it is possible to have a complete mismatch, so may be we can reveal some of our wrong choices here, just as a kind of "lessons learned". Of course, I realize that it is relative, as what may sound correct to us may not sound correct to others. Nevertheless, I’m sure some of us feel that we made a wrong choice, so who is ready to admit...
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Yes I have bought stupid stuff!                               
Worst of all was trading in a wonderful Hafler DH110 I built from a kit to buy the first Sony DSP preamp, the ta e1000esd(back around 1988?). I stopped listening to music after the novelty wore off.Took me 6 months to figure out it was the Sony killing any enjoyment of music.                          
The preamp I bought to replace it  was a used Counterpoint. Which naturally broke, got fixed, broke again. My second biggest mistake was dumping the Counterpoint, instead of getting it fixed.       
Other, more recent mistakes are too painful to write about. Some of them may not be mistakes anyway... Too soon to be sure!        
Moving the cheese can always be on and off annoying. When it turns out well, then good. When it never stops being annoying, well not good.
Elizabeth, I'm not so sure that fixing the Counterpoint yet again wouldn't also have been a mistake!  I owned a 5.1 for several years and just tired of the exploding capacitors and quickly dying tubes.  It did sound nice, though.....
I once combined Burmester 011 pre with Audio Research CD 8, it never gave me the satisfaction I was looking for. None of the equipment was defective, it just wasn't right. Not sure why, but after I had changed the components, (changed both the Burmester as well as the Audio Research) I got the level I was looking for. I find it easier to stay with the same brand, as the matching is usually working better. 
Selling a Superphon Revelation preamp & replacing it with a McIntosh. Probably the worst sounding preamp I've owned. I was young then & thought that the McIntosh would sound better because it sold for so much more. Lesson learned... Around that same time I sold a Yamaha B1 amp to go with a tubed audio research. Not that the tubed audio research sounded bad, I just with I wouldn't have sold the B1. It was a glorious amp!  
All my components (except turntable and speakers) are from the same manufacturer.....they all play together nicely.
It appears that my Acoustech phono's output impedance and passive preamp section of Redgum RGi120 integrated's input impedance are bordering the mismatch. Still sounds good. I often run the phono thru tape deck first and then to the amp. Sounds better overall with a slight drop in resolution. No other mismatches I am aware of.