When Ariel wears her t-shirt in close-up shots, her necklace switches back and forth between hanging outside the shirt and mostly hidden under the shirt.
While Heather drinks orange juice with Leonard, the amount of orange juice in her glass changes several times, and her bangs switch back and forth between disorderly and parted neatly in the middle.
When Heather sits down to drink coffee with Leonard in the morning in his apartment, they each pour coffee in their cups. Yet, when they both sit down to talk and sip, there is no coffee in either of their cups.
In the scene where Leonard goes back to apartment after stroke, as he sits in the chair there's a boom mic shadow visible on the back wall. The scene then shifts to Lili Taylor, and when it returns the shadow has gone.