Prior to 2020, the only time in U.S. history where the challenging party knocked off an incumbent president but failed to win control of Congress was Grover Cleveland's first victory in 1884.
1980?
Ugh, I'm doubly wrong here, since not only did I forget 1980, but Cleveland beat an incumbent in 1892, not 1884.
So, to try this again, 2020 would be the second election in which a party knocked off an incumbent but didn't gain total control of Congress, and the first election where a party knocked off an incumbent and neither gained a trifecta (simultaneous control of president+Senate+House) nor flipped a chamber of Congress.
It's also the third election where a party beat an incumbent but lost seats in the House, as that also happened in 1992 and 1892.