I remember in college hearing about people playing minor 'pranks' using this device, however I've forgotten the name and I can't remember the exact details.
It has to do with notes (possibly sevenths?) being raised or flattened according to whether they are being sung in a descending or ascending line; when they combine they sound 'weird' nowadays but apparently the Elizabethans (or whoever) loved it! So you can annoy your conductor/choir leader by using this device in any choral music (hilarious I know!).
But I can't for the life of me remember the name of the device, does anyone know? Thanks!