During the very end of the credits, when the film is dedicated to the memory of Donald Pleasence, they misspelled Pleasence as Pleasance.

In the beginning credits, there's a picture of Laurie Strode in the Haddonfield Yearbook. It says "Class of '78." It should be "Class of '79," however, because the film took place in October of 1978, when Laurie was a senior, so she would have been graduating in the summer of '79, making her "Class of '79."

Also in the beginning credits, one of the articles says "Lauries Strode, who survived the Haddonfield murders in 1968..." instead of 1978.

In one of the articles at the beginning of the film, during the credits, Michael Myers' name is spelled wrong. They spell it Meyers at one point.

In the opening sequence, Marion's house is supposed to have a loss of power.  When Jimmy goes to investigate, however, the light bulb in the refrigerator comes on when he snags the beer. If there is no power in the house, then the light bulb should not have come on in the refrigerator.

More than one mask was used in the filming of the film. This is quite evident as the look of the mask changes from time to time in the film.

In the scene where Jamie Lee Curtis is telling Adam Arkin who she really is, they are lying on the sofa in her living room. After she tells him, she asks if he wants a drink. She goes to the kitchen and gets a bottle of vodka from the fridge. As she is walking back into the living area, she unscrews the top of the bottle and takes a swig. Never replacing the top, she hands the bottle to Adam Arkin who (before he takes a drink) unscrews the top.

When Keri and John are arguing over Yosemite in the Kitchen, Keri slams the refrigerator door shut. A magnet falls to the ground, but when there's a cut, you don't see the magnet on the floor.

Jodi Lyn O'Keefe's name is mis-spelled on the theatrical poster as Jodi Lynn O'Keefe.

On the original theatrical poster, Janet Leigh's character was incorrectly listed as "Hattie."

The SCREAM 2 scenes on the girls' TV have continuity errors.

Although posters, advertisements and trailers only show H20 as having Dolby Digital and SDDS sound mixes, H20 was also released with a DTS sound mix for its theatrical release.

When Keri and her boyfriend are having lunch and he goes to the bathroom, she calls the waiter over to order another drink. Each time they cut to her the glass is either in her hand or on the table.

After Charlie has been killed and Sarah is in the lift with him, his position keeps changing; first looking at her then not, then he is and finally not.

When John and Molly are running out of the building, John jumps out of the window head first, but when the camera cuts back to him, he is seen coming out feet first.

When Laurie is confessing to Will who she is, she mentions that Michael murdered Judith when she was 17.   However, when Michael places the tombstone over Annie in Halloween, it reveals Judith was born November 10, 1947. Considering she was killed in October, 1963, she would actually be 15, not 17.

When the woman places the rock in front of the door, the large side of the rock is facing outward, but then when the view changes the large side is facing inward.

In the scene when Keri has just finished teaching the class, after the discussion about fate, she calls John up to give him the permission slip to go to Yosemite. As he is approaching her, there is a stand-alone chalkboard that has "Yates" (as in William Butler) incorrectly written on it. They discuss the field trip, and when it cuts back to the chalkboard, it's correctly written as "Yeats."