[1] In some scenarios, a digital watermark is added to a piece of media such as an image, film, or audio clip, to prove its authenticity.
Alternatively, a security system could be devised which would limit a user's ability to manipulate any piece of media which contained a watermark.
For instance, a DVD burner may prohibit making copies of a film which contained a watermark.
[2] In the first scenario listed above, this would allow an attacker to have an inauthentic image declared authentic, since it contains a watermark.
In the second scenario, an attacker could flood the market with content which ordinarily would allow a user to manipulate it as he saw fit, but due to the presence of the watermark, limitations would be imposed.