If your piece is a practical piece that someone is likely to be sightreading, I can say from experience that on the organ bench I would run across your double flat, commend your immense knowledge of music theory, and curse you as I fumble my way through the measure.
For organ, it's an easy one but loads of fun to play, Buxtehude's Gigue Fugue in C.
For lesser keyboards, Byrd's Carman's Whistle.
And for voice...the Gregorian Crux Fidelis for Good Friday, or the Reproaches.
