Ignorance of the law is never an excuse, but ignorance of the facts is frequently an excuse. "Innocent" copyright infringement can occur when you think it is legal to copy something (e.g. the copyright has expired, it is fair use, etc.), but it turns out that you are mistaken.

However, for copyright to apply, you actually have to copy something. If you coincidentally come up with the exact same song that someone else wrote already, but you never heard about the other person's song, you would not infringe copyright in the other person's song.

In contrast, patents infringement is "strict liability," meaning that you can infringe a patent without even knowing that the patent exists.