They sang the wrong notes and the chord is less than pleasing.
Do you have a break down? I think not.
Do you make nasty faces & tell those around you what a bad job that was? I hope not.
Do you smile & stop & say that it was a great effort but no cigar? I hope so.
How you manage the mistakes, which by the way are absolutely necessary to learning, will set much of the tone of your choir.
If you have paid singers who are expected to learn their parts on their own time and come well rehearsed, well maybe you can be cross when something isn't perfect. However. even then, you need to be thoughtful of others. Just because you (and this could be the director, conductor, part leader, or fellow singer) know what it should be, don't be sharp or ruthless in your efforts toward perfection. I always say you need to put yourself in the other person's place. When you are perfect and NEVER make a mistake then you can get all bent out of shape. Until the that second coming, here are some of my suggestions for handling the learning curves commonly called "mistakes".
1. Catch them doing something right. - We are human and no matter what we will make mistakes. Know that right from the start and keep in mind that humanity means mistakes. You might want to review a piece and decide where they might have difficulty before the rehearsal. When they do sing it AND avoid those pitfalls you get to praise them. Even if they don't avoid the wrong notes altogether, find what they did that was really well done.
2. Base criticism on what they did right - I know it sounds like repetition of #1 there is a bit of a twist here. When something doesn't come out right, phrasing, harmony, timing, etc. make certain that you start the discussion with what they did right FIRST. Then, point out the part that needs repair. They certainly did a tricky rhythm well or attacked a consonant particularly sharply. Find something right, then fix other bits.
3. Maybe the cat threw up on her shoes - Please remember that everyone no matter how well trained or motivated or willing is human. They may have had an argument with a spouse or spoiled dinner or just got bad news by phone. We all have less than stellar days. If you add to that by being negative or having a "hissie" fit over badly done triplets you are not going to have happy people. Sometimes people aren't paying attention because their life outside choir is taking their thoughts elsewhere. Be mindful always that there is more to life than that particular choir practice or performance. It does not mean you accept those wrong bits. It does mean that you don't let them be more important than those marvellous souls around you.
4. Have fun - We had a spectacular practice last Sunday. The choir worked really well and the blend, harmony and phrasing were superb. Well except when it wasn't. We got to the cadence of one piece and although it wasn't horrible, it slid into some discordant sounds. As I do my face scrunched up. I told them that that cadence sounded like an unexpected dill pickle tastes. It really did. I had trouble getting my right eye to open all the way. Now, I played it up for fun but when we went back, oh they nailed it.
So what did that do for us. Firstly, they now know how a dill pickle sounds. They know that I love them even when they are sour. They know that sour can turn sweet with a pretty quick review. Nobody's feelings were hurt and we fixed it.
Mistakes are inevitable and most often out of your control. Your reaction to them is totally controllable. My final word to our choir is that no matter what, all mistakes are mine. If I lead them astray with an entry, rhythm, words - anything it is on me. They are always great!!
Whether someone is sitting beside you or in front of you, encourage first and fix secondly. NO do not accept inferior work but work to get it to better then to best without harming relationships and while building self-worth always!! Keep your priorities in order. People are precious.