If you listen to your horse, with more than your ears, you will hear him talking to you. ~~Judy Ryder
The horse, if you take the time to notice, will be trying to communicate to you.
In this picture of an Icelandic Horse, the noseband is not too tight; that's good. But the horse is opening his mouth for a reason. It could be that the hands are not educated enough. It could be that the bit doesn't fit. Or possibly that he has a physical mouth problem that needs attention. Or any number of other things.
In any case, a good horseman will listen to his horse, and attend to any indication that things are not right.