Without knowing what your readings are, it is difficult to think about how to answer this question. Literature provides too many examples of betrayal to list here, virtually every one of Shakespeare's tragedies, for example, includes some sort of betrayal.
More generally, while I think most people are fundamentally honest, it is always best to get things in writing. Sometimes, it seems, the temptation to make a little extra money or get ahead at work can be too much to overcome. There is a difference between being suspicious and being prudent.