In US presidential elections, after electors have voted, states send electoral certificates recording the electoral votes made; these are then tallied by Congress to find the winner of the presidential and vice presidential elections.
In the 1876 US presidential election, four states (Florida, Louisiana, South Carolina, and Oregon) sent two contradictory electoral certificates to Congress. Both the Republican and Democratic presidential candidates, Rutherford Hayes and Samuel Tildern respectively, claimed victory. The Electoral Commission was temporarily formed to resolve the ambiguity. The Electoral Commission decided that Rutherford Hayes would become president in an 8-7 vote along partisan lines, and Samuel Tilden backed down and accepted this decision in exchange for ending Reconstruction in the informal Compromise of 1877.
In Will He Go?, Lawrence Douglas argues that such a thing could happen in the 2020 election, and gives various scenarios in which an ambiguity in the result could lead to the governor and legislature sending different certificates of electors to Congress.
Will any state send multiple certificates of electors following the 2020 election?
This will resolve positively if multiple certificates from one state are read in the congressional join session for the counting of electoral college votes, or if there are credible news reports that competing certificates have been sent by at least one state.