A Tool Assisted Speedrunner (TAS) by the name of ais523 has managed to beat Super Mario Bros. 3 in two seconds. To achieve this, ais523 discovered that rapidly pressing the buttons on the NES controller allowed him to insert new code into the game. The number of button presses required was out of reach of ais523 himself, he would have to press buttons 6,000 times a second.
So how did he manage to press the buttons that quickly? He called on a Nintendo robot called TASBot (a modified R.O.B. robot) to button bash on his behalf.
Using a specific pattern, the game goes from the opening scene of the game to finding Princess Peach in the correct castle, all in under 2 seconds.
The feat was carried out as part of Summer Games Done Quick and streamed live on Twitch and you can watch below:
While some could argue that the speed run of Super Mario Bros. 3 is only possible by cheating, tool-assisted speed runs are a legitimate approach. Due to that stance, the run stands as an official time for the game.
If you want more detail on how it was done, ais523 explains it over at Reddit.