"Our Last Summer," includes the lyric "that was the time of the flower power," which is usually a reference to the late 1960s. In the same scene, Harry says he traded his John Lydon tee-shirt to get a guitar for Donna. Lydon was a member of The Sex Pistols, which formed in 1975.
ABBA's iconic lyric "My my, at Waterloo, Napoleon did surrender," is incorrect. Napoléon Bonaparte retreated from Waterloo in June 1815, attempted to hold his army together for a month, then traveled to the European coast, where he surrendered to the British Navy.
During "Take A Chance On Me", Bill appears to be standing directly under Rosie as she falls from the roof since he's looking directly above him. When Bill actually catches her it's clear that she is coming from the right side of the screen.
As "Lay your Love on Me" transforms into Sophie's bachelorette party, cast members hold Sophie up and she drinks from a bottle, then throws it. After she throws it, they lay her on her back and her hair falls off her shoulders. In the following close-up shot, her hair falls off her shoulders again.
After telling her guests about the toilet, Donna goes to open the window shutter - which then falls off - with just her left hand. When the camera switches to the outside shot, Donna opens the shutters with both hands.
During one song, an extra is up a ladder, which is pulled out from underneath him, leaving him hanging from the window ledge. In the close-up, a safety wire is visible, going out of his clothes and up into the window.
When Sam, Bill, and Harry first arrive at the Villa, they all seem known about Donna's daughter. Later, before "Dancing Queen," Donna says she never told any of the men about Sophie. We can assume that Sophie's name was on the invitations.
The goof items below may give away important plot points.
When Donna kisses Sam after she says "I do," she has a broad ring on her right pinky, and two rings on her left hand (middle and ring finger). In the very next shot the ring on her left ring finger has switched onto her right ring finger. Also, her right hand changes position between the two shots.
When Sam is singing "When All Is Said and Done" to Donna, he starts out with his sports-coat on and you see the end of the coat sleeve when he reaches to caress Donna 's face. However when the camera shows him singing, he is only wearing his dress shirt and his cuffs are rolled up. Since the entire scene setting has changed between the shots as well - with Donna having her shawl now over her shoulders, and all the people previously standing around now sitting down - one can assume that these are actually two separate scenes, and the intro of the song just being laid over the end of the marriage scene.