Mistake is no excuse in copyright infringement, because there is no requirement of intent or willfulness. The copying of copyrighted material without permission constitutes infringement. As the Eighth Circuit Court of Appeals put it:
Once a plaintiff has proven that he or she owns the copyright on a particular work, and that the defendant has infringed upon those “exclusive rights,” the defendant is liable for the infringement and this liability is absolute. The defendant’s intent is simply not relevant: The defendant is liable even for “innocent” or “accidental” infringements.
While innocent intent is no defense to liability, it can result in reduced statutory damages.