I hate with that happens!!
All the male tenors were singing the second tenor part, so they put me and two other altos on the first tenor part. Sight-reading in the lower register when you're used to singing in the treble clef ain't the easiest thing on earth, I have an advantage because I play keyboard so at least I've SEEN the bass clef before -- but I had to drag the other two altos by the ears, they kept wandering back to the alto part. Lord knows I was doing that myself in first rehearsal on Thurs night, but I took the music home and played it over and over again until I had beaten the alto part out of my head. The problem with this piece (Durufle's Ubi Caritas) is that the harmony parts don't make sense - there's a lot of dissonance and parts moving against each other, so it IS hard to stay on your line. You can't stop listening to the other parts, of course, but you have to hear yours in your head and stick to it.
My husband was down in the south transept and said it really sounded good. (The acoustics in this sanctuary are great, if you like ancient music - about a seven second hang time so far as I could tell.) So everything worked out all right.