At the beginning of "Freddy, My Love," the sequined straps of the gown that Marty's wearing under her nightie are visible a moment before the big reveal.
During the "We Go Together" dance break you can hear a crew member telling them to "Get Closer!"
During the carnival scene right after the cast gets out of the golf carts when Vanessa Hudgens comes running forward she experienced a minor wardrobe malfunction when her shirt popped open and she quickly covered up before the viewers saw more than just her bra.
Toward the end, as the trams are driving to the big carnival, the second tram driven by Sandy's jock boyfriend runs over the edge of a curb, causing the tram to tilt. Coach Calhoun's face shows surprise as he rides in the front passenger side.
During the car reveal scene, after Mrs. Murdock gives the one liner "I intend to get loaded myself", two of the actors can be heard conversing about the line. One says "she does it every time" and the other responds "I know, the little one liner." The two presumably did not know that their mics were still on, and the mistake was kept in both the broadcast and later released version.
The scoreboard in the gymnasium has its "Minutes" and "Seconds" signs where the "Home" and "Visitor" signs should be, and vice versa.
One of the rides in the carnival scene is the Zipper, which was not invented until 1968.
Grease Live! is explicitly set in 1959-60. The Emergency Broadcast System is mentioned in one scene, but in real life didn't come about until 1963.
During the National Bandstand scene, the old Black and White TV the waitresses are watching have the top and bottom black bars as it was being fed from HDTV camera, and down converted.
Although the set designers went through a great effort to disguise at least one camera as a period-appropriate one in the National Bandstand scenes, a modern-day "handheld"-type camera on a tripod and operator dressed in period-inappropriate clothes is frequently visible in the back of the gym throughout the scenes.
During the Grease Lightnin' reveal before the race at Thunder Road, the box the car sits on to move it is visible. This causes prevents the wheels from touching the ground which is also visible.
In the initial East-Coast airing, during the dance-off, coming back from commercial, Vince Fontaine says "American Bandstand" instead of "National Bandstand." The West-Coast airing replaced this with an alternate take where he says the correct line.
At the beginning of the slumber party, Rizzo calls Marty "Jan"
Vince Fontaine (Mario Lopez) said "Welcome back to American Bandstand" after the commercial break. Bet Dick Clark smiled up in Heaven.