Out of tune, out of breath, losing your voice...
We've all been there before at some time, and we often have no idea when we're there.
I'm talking about being out of tune.
I've known some perfectly in-tune singers who will have an out of tune stage performance now and then. I, myself, have had an out of tune song or two now and then. What often happens is you can't hear yourself in the monitors, because the mix is done incorrectly. Make sure you speak up during the sound check so you can get a clear mix in your monitors. Also be sure your body is relaxed and you're breathing. And also importantly, you're listening to yourself and your accompaniment.
A consequence of when you can't hear yourself and you're singing louder and trying to adjust your body to be louder, (especially in Rock n Roll) is when you sing too hard and blow your vocal chords out. They heal in a day or two, but his can damage your chords permanently.
If you're singing in a healthy way, your voice will become stronger every time you sing, and you won't lose your voice or feel any pain or discomfort.
Sometimes discomfort can be something as little as a tickle in your throat, but other times you won't notice your sound isn't as pure when you're straining to be louder, to hear yourself, and you won't feel the discomfort until an entire set list later. To avoid this, turn up your monitors, and relax your body.
Make sure you aren't straining when you sing. Constantly remind yourself to let go of your neck muscles, your soft palette, and your tongue, in addition to any excess body stress. Here's a warm-up to help you sound your best before each show.
Gently chew your jaw and let out a relaxed sigh, and slowly siren downward, lightly, on "yahhhhhyayayayaaaa" with a numb, relaxed tongue. Sigh through your note ranges, in a light, not-trying-to-be-loud-or-sexy kind of tone. Much like a little boy. Make sure you're breathing in a relaxed manner, and you're filling your diaphragm up without your shoulders rising. It's good to randomly sigh different parts of your range throughout the day.
Bend over and hang your torso from your hips, letting go of every muscle from the waist up. breathe in, laugh if you want, sigh. Shake it all out. Wind upwards, one vertebrate at a time. Once you're relaxed and balanced, sigh again.
Focus on relaxation, above anything else, when you're going to sing. Relaxation is the key for any talented vocalist to succeed. Take the time to meditate before your shows. Take some time to stop caring about what anyone thinks but you. Take some time to accept and love yourself. Treat yourself to something good.
When you're performing in front of people, don't think about the people, focus on your comfort, and ensuring you and the audience, can share this comfort. If the audience is uncomfortable with you at first, tell them a joke to get them to relax. If you relax first, and keep up the relaxation, the audience will relax and remain in their seats. If you're tense, the audience will reflect your tension. Audiences are mirrors.
I hope this article was helpful to you! I know I certainly have good nights and bad nights, though less bad nights these days.