To play the original Atari 2600 version of Pitfall, press Select, A, A, A, A, A, A, Select, Start on the title screen.
Lose your life in the first level. When the Continue screen appears, press B, B, B. The continue counter will switch to '9' and stop counting down, giving you infinite continues.
At the title screen, press X, Select, A, Select, Y, A, X, Select to get a level-select option. The name of each level appears under the title. Press L + R to scroll through the levels.
Go to the Run Away Mine Car level and at the end go to the bottom track and jump into the door on the middle track, this will start the game over.