Walter Meyer quoted Peter Schickele:

>Stravinsky was present at a recording of his Septet ...  the clarinetist
>realized to his horror that he had just been playing the wrong clarinet
>(A instead of Bb, or vice versa) throughout the movement.  He had been
>off key the entire time and hadn't noticed it.  Hurriedly he explained it
>sotto voce to the recording engineer, who hadn't noticed it either.  They
>explained to Stravinsky, who hadn't noticed the off key clarinet either
>(!)...

I can believe it.  I also have heard that there was a chamber work by
Elliott Carter that was published with one of the trumpet parts scored for
the wrong key trumpet.  The work (I don't remember which one) was recorded
(I don't recall by whom) from that score, and not even Carter noticed that
the trumpet part was in the wrong key all the way through.

If there is a moral to be derived from this, it's a warning to composers
to question themselves lest they find themselves writing beyond what
they can hear in their head.  It might betray more of a commitment to an
intellectual notion than musical beauty.  Or perhaps it's just hard even
for geniuses to remember every little thing they've done before.

I speak as an admirer and devotee of both Stravinsky's and Carter's
music, by the way.

 - seb