Believing something untrue in a dream is read as a question about what you are not seeing clearly. Dream books link it to illusions held about a person or a plan.
Has this come up in your dreams before? What was going on in your life then?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.