Copyright infringement can be costly, particularly when the work of one artist is used without permission in the successful work of another. A lot of ink has been spilled on famous - and infamous - cases of infringement that have resulted in the repayment of royalties, injunctions and shared copyrights. Sometimes infringement is avoidable (Google is widely available), and often it has been found to be unintentional. Even so, under most circumstances, an unintentional infringer may not continue to profit from the work of another.
Whether infringement was done knowingly is a matter that is to be determined by the fact finders in a case - the judge or jury. A knowing infringer may be required to pay more in the way of damages than an unknowing infringer. In some cases, intentional copyright infringement may even rise to the level of being a crime.
Often the amount of one's work that is used in the work of another will help to determine the extent of an infringer's culpability. For example, pop icon and former child actress Miley Cyrus is currently embroiled in a dispute over seven words in one of her songs. Her 2013 tune "We Can't Stop" uses lyrics that are almost-verbatim in similarity to a 1988 song written by Jamaican songwriter Michael May. In addition to the seven words, May claims that the song is similar in melody and rhythm to the tune he wrote.
May's attorney has said that the infringement is worth $300 million. Whether there actually was infringement and whether it is worth what May claims has yet to be determined. But the relatively small amount of lyrical similarity will certainly be noted by the defense - which should be mounted by an experienced copyright attorney.
Source: ABC News, "Miley Cyrus sued for allegedly stealing 'We Can't Stop' lyrics," Andrea Tuccillo, Mar. 14, 2018