Continuity mistake: In the pre-credits sequence, when Danny and Sandy on walking on the beach, Danny is on the left and Sandy is on the right. However, in the next scene, when they kiss, they have suddenly changed positions. (00:00:40)
Continuity mistake: During the song Summer Lovin', Rizzo lies down on her side on a picnic bench and puts on sunglasses. In the next shot, when she kicks Sandy and another girl off the bench, the sunglasses are off. In the following shot, they are on again. (00:16:13)
Continuity mistake: Sandy walks over to meet the girls after doing a cartwheel. On Sandy's cheerleading uniform, there is a LONG red string right next to the letter on her shirt. Then, suddenly, the string disappears in the very next shot. Then in the scene after that it is back again, and then the string is back on her shirt. (00:22:20)
Continuity mistake: During the slumber party at Frenchie's, when Rizzo brings out the wine, check out the junk on the headboard. Later in the scene when Marty is putting on her robe, the headboard is clear and now has a black fan on the left. Then even later when Jan says "You ain't getting your hands on my ears" the headboard is cluttered again, with the black fan too. (00:27:35)
Continuity mistake: During the Greased Lightning song sequence, when Danny runs around the dream car, from one shot to the next, 2 white mattresses suddenly appear behind the car to aid the dancers jumping off. (00:39:55)
Continuity mistake: In the Greased Lightning dance scene, look just in front of the last white desk on the right side during the dream car segment. One of the dancers knocks a tyre off a stand and it falls in front of the desk. A shot or two later, the tyre is back up on the stand. (00:40:05)
Revealing mistake: When Sandy is at the shop with the jock, then she goes to the jukebox, the picture behind the jock is all blurred out. You can tell it is a Coke sign - Coca Cola cancelled their sponsorship after filming. (00:43:30)
Continuity mistake: When Kenickie says to Rizzo in the soda parlour, "How about if I finish with you, huh?" there is a tomato ketchup squeezer on his side of the table. In the next scene when Rizzo says, "Finish this!" and throws the glass of liquid at Kenickie, the ketchup squeezer has moved to her side of the table, and in the next scene, when Kenickie is covered in liquid, the ketchup squeezer is back in its normal position. (00:52:00)
Revealing mistake: In the soda shop, the waitress turns off the lights with her elbow because her hands are full, but she misses the light switch by 6 inches. (00:55:10)
Continuity mistake: At the beginning of the National Bandstand scene you see people filling the hat of the big "Rydell Ranger" figure with pieces of paper. During the next ten minutes the figure is often visible in the background and the hat is empty - nobody emptied it. After those ten minutes the hat has paper again and is then emptied. (00:59:00)
Visible crew/equipment: In the scene during the American bandstand, at the end of the song when the teacher is dancing in front of the band members, the crews can be seen. The dressing and the hair doesn't match Grease era at all, and someone is sitting on a director's chair as well. (01:06:20)
Continuity mistake: Once during the dancing scene you can see Vince Fontaine standing on Marty's left side, and then suddenly on her right. (01:10:16)
Continuity mistake: When Cha-Cha receives the award from Principal McGee at the end of "Born to Hand Jive" at the National Dance Off, Putzie is giving Jan a piggy-back next to the Principal. However, in the next scene, Putzie is giving Jan a piggy-back right next to Cha-Cha. (01:14:00)
Continuity mistake: After the Song "You're the one that I want" when Danny is doing the test your strength game you can see Frenchy running from the Ferris Wheel a second time when Rizzo and Kenickie start kissing again, when that scene happens just before the song. (01:44:50)
Factual error: In the dancehall, the guitarist in the band is using a silverface Fender guitar amp. Fender did not manufacture this style till early-mid 60's. (02:00:00)
Factual error: The movie and events take place in 1958. However, two Fender amplifiers that are used by the band, "Johnny Casino and The Gamblers" are "Silverface" models that were only manufactured between 1967 and 1981.
Audio problem: During "We Go Together" at the end, John Travolta's mouth is not in sync with the words "we'll always be like one." He appears to start singing "that's the way it should be" before realizing and correcting his mistake.
Visible crew/equipment: In the soda shop scene where Sandy and Danny are at the jukebox, the reflection of the boom mic is clearly visible in the chrome part of the jukebox. [True of videos and early DVD releases - in later DVD releases the reflection has been removed].
Audio problem: At the cotton candy stand at the graduation carnival, Blanche is seen silently mouthing the question "How many?" In the next scene, from a different camera angle, the same question is asked - this time with audio intact.
Continuity mistake: When the three T-Birds go to moon at the TV camera at the National Dance Off, Doody says "1, 2, 3, go!" and pulls down his trousers to his ankles. In the next scene, his trousers are suddenly pulled right up to his buttocks.
Answer: Well, Danny was out of town, at the beach. It's quite possible that the rest of T-Birds didn't live close enough to each other to see each other during the summer. Also, they may have all had jobs, or been travelling with their families. It's also possible that they DID see each other, although maybe not as "T-Birds". When they saw each other on the first day of school, though, they were Seniors, and they were ready and excited to be T-Bird seniors.
BGraz
Another possible reason is that not all of them were allowed to see friends during the summer. I had a couple of classmates whose parents were strict about them focusing on schoolwork and/or getting a job. No phone calls or visiting friends on the weekends or during summer: "You can see your friends at school."
Do you really think any of the T-Birds had strict parents?
People quite often rebel against strict parents.
When they asked Kenickie where he was, he said "working, which is more than I can say for any of youse kids" suggesting that the 3 stooges (pun intended for their stupid routine that prompts Danny to tell them to "be cool") didn't work all summer. Also, Sonny needed to borrow money in the dinner until he could get his allowance.
In regards to not living close enough to each other, it is worth mentioning that having access to a vehicle was much less common compared to nowadays.
KeyZOid