That's not true, it's happened before. The 1876 election for instance was highly contentious and I believe the first time a candidate won the popular vote without winning (Samuel J. Tilden from the Democrats). The election was disputed over a number of things regarding voter fraud, including accusations of disenfranchising black voters. Rutherford B. Hayes ended up winning through a compromise with the electoral commission, in what's called the Compromise of 1877.

Basically, democrats agreed to cede victory to the republicans if union troops were pulled out of the south and reconstruction ended. This was honestly a very shitty idea because it directly led to Jim Crow laws and the structure of segregation that would last until the 1960s.

Hayes was popularly regarded as a fake president in the south, only given the presidency through backroom deals instead of appointment by the consent of voters. The newspapers called him "Rutherfraud B. Hayes" which is honestly a pretty good dunk on his name