"I know where a fake coupon is being distributed. What should I do?
If you know the people involved in distributing the coupon, it is not worth the risk to confront them. The Coupon Information Corporation (CIG is a good source for reporting a coupon that you think could be a fake. If the coupon was mailed to you, keep the envelope and any other paperwork.
If it is a single online coupon that is printable send the link to the CIG.
If a couponing site that you use has linked to the fraudulent coupon, use their contact information and let them know. Chances are that the coupon was listed in good faith to help other couponers and the site owners are not aware that it is a fake. They will want to know so that they need to remove it from their site.
What you should not do is ignore the situation. Fake coupons ultimately cost the stores, the manufacturers and all coupon clippers because a loss could mean less coupons are distributed by the companies involved.
See also: Couponers Can Help Fight Coupon Fraud