If you tell a student that he or she will pass your class and then you change your mind and fail the person, that would be a major deal. Little things matter too, though. Let's say that you tell your students that they will have no homework for the weekend and then you decide at the last minute that they need some. This may not seem like a big deal to you, but it could be to them.
On the rare occasions where I feel like I have no choice but to break my word to my students, I try to be sure to apologize as much as I can and I beg them for forgiveness. I might offer them some kind of peace offering, like candy or no homework later or something like that. I realize how big a deal it is to break my word to them, and I try to make up for it.
So be careful. Make sure you keep your word to your students as much as possible. Do it for the small things as well as the big. You will have a much better chance of keeping their respect if you do.