There are legitimate or at least understandable cases when one may hear a familiar piece of art music in a key different from the one it was originally written. In some cases transposition was done by the composer to accommodate the capacities of different instrumental or vocal ranges (e.g., Bach, Rossini).
In other cases transposing was done by generations of musicians who considered playing the piece in the original key unreasonably difficult or cumbersome (e.g., Paganini's First Violin Concerto). Or because some musicians wanted to play works they love but the range of their instruments could not accommodate the work in the original key (e.g., the French violist Antoine Tamestit's concert performances of Bach's D-minor Partita for unaccompanied violin.)
Still other cases of transposing are traditionally accepted acts of charity toward aging (or under the weather) opera singers for whom a semitone difference can be the difference between a credible performance and a humiliating, traumatic disaster.
Then there are the artifacts of early recording technology when the lack of universal standards for recording and/or playback equipment (78 RPM shellac records, early wire and tape recorders) had resulted in recording of familiar pieces whose perceived key could differ by a semitone or more from the original.
Two more things to mention for the sake of completeness. First, the experience of hearing familiar music in a wrong key may have no causes external to the listeners mind. Instead the experience may be due to some kind of neural degeneration of or damage to the relevant part(s) of the brain (In his very late years the great Russian pianist Sviatoslav Richter suffered from just this affliction. While his fingers played in the correct key, the music he heard in his mind's ear sounded distressingly sharp by a semitone.)
Finally, the question of what key a piece of music is being played in does not have an objective answer (the way questions in physics or mathematics do). The reason is that a key depends on the more fundamental notion of pitch, and the standard determining what counts as a particular pitch not only has changed dramatically (even ridiculously) over time but remains to some extent 'fuzzy' even today.
Did Bach play his A-minor Violin Concerto in the key of A-minor? If so, then today's performances by star violinists and top modern instrument ensembles must be playing this work in B-flat minor. After all, the frequency of the tuning pitch A (above the middle C) has gone from about A=418-420 hz in the time of Bach and Handel to today's A=440-444 hz (and higher!), which makes modern performances a semitone higher.
In what keys exactly did Beethoven 'play' his works in his mind's ear, given that the turning fork he kept (now in some British museum) was set at A=456 hz! This, again, is a semitone higher than the A=430 hz used in the fist half of the 19th century, and today it still sits roughly in the middle between A and B-flat.
What about (admittedly exceptionally rare) music-lovers with hyper-acute absolute pitch (my brother is one) for whom the practice of some orchestras to tune way above the so-called 'Stuttgart Standard' A=440 (as high as A=448 in Vienna and A=450 in Moscow) produces not just more 'brilliant' orchestral sound, but makes the music sound perceptibly off-key compared to performances of the same music by other orchestras?
All of the above, as I said, are cases that are at least understandable. Recently, however, I came across a case that I simply can't understand; and being unable to understand it turned out the be the least of my problems. The case in question is a 2017 concert performance of Liszt's Second Piano Concerto in A major (S.125) by the distinguished French pianist Jean-Yves Thibaudet with Orchestre National de France conducted by Emmanuel Krivine. This performance was recorded for broadcast, and when the recording was made available on-demand as a 256 kbs mp3 stream, I simply downloaded the streaming source file (so no actual streaming and/or re-recording took place on my part) and proceeded to listen to yet another interpretation of a work I've loved for years despite a few minor reservations. What I heard, however, was a piano concerto in B-flat major, the key which, as far as I know, Liszt never authorized in print or privately as an alternative to the key in which the work was published.
