After Josie eats the brownie, she dances with the band and everyone is laughing, including George back in the van. However, the video/pin is on Josie's dress, therefore he would not see her dancing, and instead would see what Josie sees.
Josie Geller is 25 when she enters the senior class of 1999's Prom. This would have made her, if she graduated at the age of 17/18, a graduate of the class of 1991/1992. Her prom dress flashback is a 1980s outfit. She was invited to the "senior" prom, which means her date would have had to be a senior or junior to invite her, and since he was her classmate, she would also have been a junior or a senior. So, she would have graduated during the 90s, yet her prom dress is in 1980s style, and any flashbacks of her "Josie Grossie" lifestyle show 80s outfits and play to Cyndi Lauper's "She Bop" - a hit in 1984, 8 years before Josie would have graduated, and at least four years before she was old enough to be in High School.
Before the prom after Josie and Guy show off their outfits, in particular Guy's sword, you see as everyone walks into the prom, you hear Rob say, "I wanted a sword," but he is clearly smiling and not saying anything at all.
After George puts the hidden camera pin on Josie's sweater and she gets out of the van, we see George sit down and reach up to the monitors with no headphones on. In the next shot, he's suddenly wearing headphones.
After Josie gets the hidden camera pin, we see an almost head to toe shot of her walking up to her locker. After Guy and his friends go by, Josie walks up to the kid with fliers and we see that there is something now wrapped around her left ankle/leg.
When Josie and Sam are painting, she wipes paint on his face. In the video that everyone at the office is watching, he wipes the paint off before putting paint on her face. In the shot of them "actually" he doesn't wipe the paint off at all.
When Josie is telling the gym teacher she needs water, her hands are on the teacher's shoulders. In the next shot (from the opposite direction), Josie's arms are slack. Then, in the next shot (from the original direction), Josie's hands are on the teacher's shoulders again.
When Josie is asked to read from Act 5, Scene 2 of "As You
Like It", she opens the book to the very beginning of the play instead (although it is conceivable that she was reciting from memory and didn't need the text at all). Also as Josie's book it open to the beginning, Aldys' book it open to almost the end, and the girl shown behind Josie book it open to the middle.
When Sam turns up at the baseball stadium and various characters begin kissing each other, Kristin hits Jason with her purse when he tries to kiss her. After this shot, they can be seen in the background performing the same actions.
When Josie enters the reggae nightclub, the word "DELLOSER" is stamped on her hand. The top half of the stamp has the word regular the bottom half has the same word upside down. It still reads from left to right but the letters are upside down. Later that night, when Josie falls asleep on her hand, some of the ink from the stamp gets on her forehead. Her forehead says "Loser" - forward and right-side up. If the upper portion of the stamp had transferred, it would have said "Loser" backward. If the lower portion of the stamp had transferred, it would have said "Loser" upside down and backward.
When Gus and Anita voice their concerns about Josie's assignment, she says, "Neither of you think I can do this." Josie, a noted pedant, would know that when used as a pronoun "neither" is singular and would say, "Neither of you thinks I can do this."