CREDITS
Go to Special Features, then click on Otnemem.
On the newspaper clipping, move to the upper right corner and an arrow (triangle) pointing to an exclamation point will be revealed.
Select this, and you are asked Who did I kill? Select the right person and the DVD production credits are displayed.

HIDDEN VERSION
Access the Special Features section of the DVD.
Wait for the Memento Mori picture to move and the pic of Leonard with a pic in his mouth to take up most of the screen. Press Enter.
A screen will come up telling you you have found the hidden version of the film. Click on Play.
Watch the film in chronological order including the end credits rolling backwards at the beginning and the opening segment running forwards