Tonal memory can work against us as well, however. Singers have impressive abilities to memorize where pitches are.
On the positive side, Robert Fountain used to do an exercise in tonal memory with an Eb Major chord (from the bottom up: root, 5th, root 3rd), asking his choirs to be able to produce it from memory at any time. I've known other conductors who've worked on that kind of memory (not perfect pitch, but to develop a memory for a particular chord or pitch).
When I'm working on a piece intensively, I almost always find that if I don't think about it, but simply begin singing it, I'm almost invariably in the correct key (I don't have perfect pitch). My choirs can often do this as well.
But it also takes little time to memorize pitches incorrectly. An example:
With my PLU choir I did John Gardner's wonderful and dramatic, A Latter-Day Athenian Speaks (published by Oxford, now available only on rental, it's a fabulous--and difficult--a cappella setting ca. 13 minutes long). We'd been working on it, preparing for a January-Term tour to the mid-west and east coast. The end of the piece has a dramatic double-choir fugue and, even though we'd been singing it well in tune through the rehearsal process, when we got to the first performance, the choir (with all of the energy and excitement that goes with a first concert) drove that section of the piece a half step sharp. After that, we always sang it sharp. I'd rehearse it with some reference pitches from the piano and they'd lock it in, but in concert they'd be a half step sharp within very few bars. In essence, they now memorized going sharp there, heard the opening of the fugue that way, and no matter what I did, that's what was going to happen.
The power of tonal memory is just that strong.
It's one of the reasons that you have to be very careful not to allow your choir to flat or go sharp early in the learning process--it quickly becomes a part of how they hear the music and tonality. It's a reason to listen carefully early in the process. It's also why rehearsing well, not trying to do too much too soon, or using Robert Shaw-style count-singing/rehearsal techniques, etc., can make a huge difference in whether your choir stays in tune or goes flat (more usual than sharp, of course).
It's also the reason why, if your choir has been going flat in a particular key, if you suddenly raise the pitch by a half-step, they may be able to keep it in tune: you've moved out of the tonality where they've memorized going flat. They can now approach it with a fresh sense of where those pitches belong.
It's not about listening! Sometimes we say, "Listen!" . . . well, how could they go a quarter-step flat, exactly together, unless they were listening to each other?!
I'll write next about some ways to rehearse to avoid these kinds of problems. The use of the piano in rehearsal is a part of that.