Livre d'Or

And the promised operatic bass was sick so they had a last minute substitute, not even an understudy but another artist who just happened to know the music. He was very impressive, you'd never have known that he wasn't the person who had rehearsed the work.

Unless it was an actual staged production (with acting, etc) that's not particularly unusual in professional classical singing - most artists will have a selection of repertoire roles which they have prepared and can sing on basically zero notice because they already know the piece, and you don't normally work with the orchestra or conductor until the day before the concert or even the concert day itself, so slotting in last minute is actually kind of trivial. You weren't expecting to sing it today, but you did it three months ago in Milan and have a booking for next year in New York! Nobody has understudies for concert performances, as far as I know, you just ring around the agencies and ask the artist managers "who do you have who is free tonight and can do [piece]" and pick the best match.

And three psalms, including the most amazing setting of Ps 150

Was that the actual Symphony of Psalms, or something else? We've done SoP a bunch of times, it's very cool (but hard to sing!).