(at around 2 mins) The close up shots of Claudette singing show her legs crossed, but the long shots show her legs spread apart.
(at around 30 mins) Ned is eating a hamburger when he runs to Alice's aid, but the hamburgers aren't cooked until the next scene.
(at around 22 mins) When Annie trips in the woods and backs up against the tree, her hair and the amount of leaves in it changes in every shot.
(at around 1h 9 mins) When Alice is alone making coffee, on a table behind her is a mixer and a basket of eggs. When Alice returns (at around 1h 13 mins) the basket and the mixer have switched places.
(at around 20 mins) Although the Jeep is traveling at a high speed Annie rolls only once and she gets off after she leaps from it.
Annie can be briefly heard taking a gulp of breath even after her throat has already been slashed by Mrs. Voorhees.
Brenda uses the communal bathrooms rather than the sink in the room because of facilities available there other than a sink.
(at around 49 mins) Outside the diner, it is obviously a fire hose being sprayed back and forth to create the "rain."
The lightning on Marcie and Jack is merely a yellow light being flashed at their faces; real lightning has a bluish tint.
(at around 20 mins) In the shot of Annie jumping out of the jeep, the vehicle has clearly slowed down somewhat to allow her to jump safely.
(at around 25 mins) When Ned pretends to drown and Brenda swims out to rescue him, they both stay underwater for a few seconds, waiting for their cue to emerge.
(at around 40 mins) When Brenda rolls the dice to start the strip Monopoly game, she says she rolls double sixes and gets to roll again, but you can clearly see the 1 and 2 she actually rolled. The same for the guy, who clearly rolled a 10, though it's said that he rolled an 8.
Many early prints of the film feature a large, black line running from the top to bottom of the frame when Marcie is checking the showers. It has since been digitally removed in recent DVD/Blu-ray releases.
When Bill and Alice go into the office and try to use the pay phone you see that the phone cord has been cut. However the amount of cord hanging down is far too long if the cord was running straight across the wall, as the rest of it is.
In the French translation of the movie, Mrs. Voorhees calls her son Jackie, even though the character is named Jason in the English original version. The translation was corrected in Friday the 13th Part 2 (1981).
When Annie's throat is cut, the skin around the bleeding slash wound is an obviously different color from the rest of her neck.
(at around 2 mins) When Claudette and the counselors are singing in the beginning, the music continues after she stops strumming the guitar strings.
When Alice is lying down on the couch and is having a conversation with Bill, we see neither her, or Bill's mouth move.
(at around 7 mins) When Annie walks over to the dog and starts talking to it, her mouth is out of sync when she says, "Excuse me...".
(at around 21 mins) When Annie jumps out of the jeep, a white pad on her right elbow is visible.
(at around 18 mins) When the jeep driver pulls over to pick up Annie, a man's arm is reflected in the front shield.
(at around 8 mins) Reflected in the window just below the reversed Coke sign when Annie enters the coffee shop.
(at around 27 mins) After Bill has killed the snake and Alice asks, "Is it dead?", the shadow of the boom microphone is visible moving over the door behind her.
(at around 10 mins) Annie tells the truck driver she will cook for 50 kids, but later (at around 19 mins) tells the jeep driver she doesn't like to call children "kids".
As Sgt. Tierney is giving Steve a ride back to Camp Crystal Lake, he says "It's not bad enough to have Friday the 13th, we've gotta have a full moon too. We keep statistics. We have more accidents, more rapes, more robberies, more homicides, more of everything when there's a full moon. It upsets people. Makes them nuts.". In June 1975, the full moon was not until the 23rd-24th.
If, as the film indicates, Mrs Vorhees is under Jack and Marcie's bed while they're there, she missed an opportunity to kill them both at once.