Common legal defenses to copyright infringement include:
- Too much time has elapsed between the infringing act and the lawsuit (the statute of limitations defense).
- The infringement is allowed under the fair use doctrine. (For more information, see Nolo's article The 'Fair Use' Law: When Use of Copywrited Material is Acceptable.)
- The infringement was innocent (the infringer had no reason to know the work was protected by copyright).
- The infringing work was independently created (that is, it wasn't copied from the original).
- The copyright owner authorized the use in a license.
If someone has good reason to believe that a use is fair -- but later finds herself on the wrong end of a court order -- she is likely to be considered an innocent infringer at worst. Innocent infringers usually don't have to pay any damages to the copyright owner, but do have to cease the infringing activity or pay the owner for the reasonable commercial value of that use.