What happened in the election of 1800 that did not happened before?

This is from the Library of Congress website, so put it in your own words. The short version is that it was basically a tie vote that had to be decided by Congress.

“1800 Presidential Election Results

Democratic-Republican Thomas Jefferson defeated Federalist John Adams by a margin of seventy-three to sixty-five electoral votes in the presidential election of 1800. When presidential electors cast their votes, however, they failed to distinguish between the office of president and vice president on their ballots. Jefferson and his running mate Aaron Burr each received seventy-three votes. With the votes tied, the election was thrown to the House of Representatives as required by Article II, Section 1 of the U.S. Constitution. There, each state voted as a unit to decide the election.

Still dominated by Federalists, the sitting Congress loathed to vote for Jefferson—their partisan nemesis. For six days starting on February 11, 1801, Jefferson and Burr essentially ran against each other in the House. Votes were tallied over thirty times, yet neither man captured the necessary majority of nine states. Eventually, Federalist James A. Bayard of Delaware, under intense pressure and fearing for the future of the Union, made known his intention to break the impasse. As Delaware’s lone representative, Bayard controlled the state’s entire vote. On the thirty-sixth ballot, Bayard and other Federalists from South Carolina, Maryland, and Vermont cast blank ballots, breaking the deadlock and giving Jefferson the support of ten states, enough to win the presidency." (Source: Today in History, February 17)