The date when Helen drinks the potion (October 26, 1985) is the "present" date in Back to the Future (1985), also directed by Robert Zemeckis.
In the scene where Helen sits down onto a shovel handle, she didn't sit in the way she was expected to do, so the SFX people had to morph the image to make it look like the shovel handle was pushing up into her chest.
In the scene where Madeline punches in the entrance code and enters the gate as Helen slips past her unseen, you get a brief glimpse of her license plate. It reads "2HYE305", not "ZHEMEKIS" as previously noted
A pneumatic bra was built to create the effect where Meryl Streep's breasts become higher and firmer after drinking the potion, but the effect didn't look realistic enough. In order to get the shot Meryl Streep's dresser stood behind her, out of sight of the camera, and pushed her breasts into position.
An edited picture from the original ending was used during Ernest's funeral scene. The picture of him aged was actually a of him in full age makeup from the first ending. 'Bruce Willis (I)''s aged face was used, but put on an actual picture of a mountain climber, which thus resulted in the end picture of Ernest at his funeral.
During the description of the plot to kill Madeline, there was a quick shot of the folder being stamped "case closed" at a desk. Also on the desk was a brain in a glass jar labeled "abnormal" - a tribute to the original Frankenstein.
In an interview Meryl Streep revealed that she assumed the role of Helen (Goldie Hawn's character) was meant for her. Not the "song-and-dance" role of Madeline.
The three main character's names are a play on words. Madeline, Ernest and Helen can be shortened to Mad, Ern, Hel, or "Madder 'n Hell."