The first movie establishes that Donna hooked up with her three suitors in the following order: Sam, the evening before July 17; Bill, the evening before August 4; and Harry, August 11. In the flashbacks of young Donna, she has sex with Harry first, then Sam, and then Bill.
During the Mamma Mia! sequence in Mamma Mia! (2008,) young Harry is shown with a bolted necklace, longer hair. Young Bill had long blonde hair and eye tattoo on his knees. Young Sam had long brown hair and a goatee. Mamma Mia!: Here we go again gave completely different looks to the new actors playing the young versions of the characters.
The events in both "Mamma Mia!" and the sequel take place primarily on the (fictional) Greek island of Kalokairi, but the outdoor scenes were visibly filmed in areas that look very little alike. While the original was filmed on location in Greece, on a heavily treed island, filming for the sequel took place in Croatia instead, on an island in what appears to be a much more barren climate.
The young Harry, Bill and Sam were not dressed or made up as they were in their photos as shown to Sophia on Bill's boat in Mamma Mia.
In Mamma Mia (first movie) Tonya and Rosie met Sophie's Dads for the first time. In Mamma Mia 2 Here We Go Again they meet as twenty-somethings.
At Oxford, graduation ceremonies take place in the Sheldonian Theatre, not in the colleges. Students also do not give speeches at the ceremony and there is no concept of valedictorian in the UK.
In the first movie, Donna states her mother is already dead.
At the beginning of the film Sophia is writing invitations for the reopening and writes one for each of her 3 dads. She should only have written 2 as Sam was already at the hotel.
Donna graduates from New College in 1979. Women were first admitted to New College in October 1979, so she couldn't have been graduating.
The movie says 1979 at the beginning. Donna's first meeting with Bill, Harry and Sam take place in 1973. Sophie, being 25 would mean that the present timeline of this film would take place in 1979, which means the characters wouldn't have the technology of 2017, most notably the iPhone models that Sophie and Bill's mom possess.
While no specific dates are mentioned in the first movie, Sophie states in it that she's 20 years old, and Sam comments that he hasn't seen Donna in "over 20 years." The sequel ostensibly takes place five years later, with flashbacks to Donna's youth in 1979 and the birth of Sophie in 1980 (summer 1979 + nine months), which infers that the "present-day" scenes must be in 2005. However, multiple scenes feature technology contemporaneous with when the movie was filmed in 2017, including late-model phones with large touchscreen. Most noticeably, Bill is shown driving a Mercedes-Benz X-Class pickup truck to the airport in Stockholm; it was the manufacturer's first pickup truck, and it first debuted in 2017 for the 2018 model year.
Donna wears a mortar board to graduate. Women at Oxford did not wear the mortar board until much later. In 1979 they wore soft caps.
The village band playing in a rural Greek bar/taverna in 1979 were kitted out with multiple hand-held radio microphones. Although they were first introduced in this year by Nady, professional international acts of this period were still using wired microphones and it is unthinkable a local group would be using these.
During the 14th musical number (Fernando), just after Ruby Sheridan (Cher) has said ''Fernando?'' the camera quickly switches to a distant view of Fernando (Andy Garcia) where you can see a crew member - wearing headphones - taking a fire torch off Fernando.
The one and only scene in Paris is actually shot in Bordeaux, at the Place De La Bourse. However, an Eiffel Tower has been added in the background.
Donna says her mother's heart was broken after a romance in Central America. Ruby's romance with Fernando happened in Mexico in 1959. However, Mexico is in North America. At one point, she even sings about crossing the Rio Grande River into the US.
Andy Garcia is wearing a wedding ring in his first scene, and afterwards he is not.
When Bill's twin pretends to be him at the award ceremony he says that he has a wife; however, he does not, since Bill had been with Rosie. Bill's twin may have been married, but Bill wasn't.
While Bill's talking to his mother on the phone, they're both speaking English. However, both being native Swedish speakers, the conversation should've been in Swedish. While his twin brother's speech being in English, which could be justified as everyone present at the award ceremony might not be Swedish-speakers, the bit between Bill and their mother was a private conversation between two native Swedish-speakers. Hence, there would be no reason for them to be talking in English.
In the church scene, Donna appears in the form of a ghost that only Sophie can see. However, Ruby turns her head, looks directly at Donna from top to bottom and stares for a little before the camera switches.
Young version of Donna (Lily James) has dark brown eyes, but adult version of Donna (Meryl Streep) has bright blue/green eyes.