On May 20, the South Dakota Secretary of State informed the press that Clayton Walker, independent candidate for U.S. Senate, had been removed from the general election ballot because a Democratic Party official had challenged his petition. Apparently in South Dakota, when a candidate’s petition is challenged, he or she is not even notified about the challenge until it is over.
In April, Walker had been told that he was on the ballot. See this story. Walker did not know about the challenge until a newspaper story reported the news.