A forgery in a dream signifies the deceitfulness of the dreamer. Forgery can represent a lot of different things such as deception and dishonesty by lying to others and even yourself, insecurity by trying to hide who you really are, and cheating on a test or in a relationship.
If you were to see a forgery that has been created by someone else, it may mean that your competitor or adversary was trying to deceive you and make you believe they are somebody they are not.
If you have created a forgery in a dream, it may signify that there is some part of yourself which you try to hide from other people. And this part of yourself may be your weakness.