Continuity mistake: When Sophie is finally dressed for her wedding, the gauzey material is predominately over her right shoulder/breast. Next shot it is over her left. When she gets on the donkey, it is completely off her right side and her bra shows.
Continuity mistake: During the song "Lay All Your Love on Me" on the beach, when Sophie lies back and then Sky is on top of her, in this closeup we see Sophie's wearing something else over her swimsuit. It's visible below her waist and as she lifts her right leg. (00:45:00)
Continuity mistake: During "Money, Money, Money" Donna walks into the kitchen and all the staff are sitting at the table. They hurriedly get up and start working, and a newspaper is left right beside the large fish on the table. When the shot goes back with the table visible, the newspaper has gone. (00:17:35)
Continuity mistake: Soon after arriving, Tanya and Rosie are at a bar and suddenly Donna comes out in tears. Tanya is just receiving a cocktail from Pepper, the bartender. Rosie and Tanya then hurry after Donna - and during the rest of this scene, the amount of drink in Tanya's cocktail keeps lowering then rising. (00:29:45)
Continuity mistake: Just before the song "Does Your Mother Know" on the beach, the items on the bar change their position during the scene, from shot to shot. For instance, the bucket of ice, Tanya's bag and drink, etc. (01:09:25)
Continuity mistake: During the wedding scene, all three of Sophie's possible fathers stand up and gather behind her on the aisle. In one closeup shot Sophie turns around to say something to Sam - in this shot the right part of her hair is behind her right shoulder. In the next shot as she is turning around back to face the priest, her hair is suddenly neatly in front of her right shoulder. (01:25:50)
Continuity mistake: When Rosie and Tanya follow Donna into the bathroom, during the "Chiquitita" scene the flower pot on the windowsill moves back and forth from the left side to the right side.
Continuity mistake: When they are singing Does Your Mother Know You're Out, a man is seen getting out of water and then lying in the sand. When he stands up no sand sticks to him, which it should. (01:09:00)
Continuity mistake: When Tanya and Rosie arrive at the island, they get off the boat and walk up the pier. They break into song with Donna (who is walking towards them) but as the shot keeps changing, the two old men standing on the pier disappear and re-appear every time the camera changes. (00:11:25)
Continuity mistake: During 'Winner Takes it All', at the start of the song, Donna has her hands up near her chest, in the next shot they are down by her side.
Continuity mistake: During the last half of the movie Donna wears a red sweeping shawl over her shoulders, with her outfit for Sophie's wedding. Many times the position of the shawl is not consistent during shots - most noticeable after she walks Sophie down the aisle, then sits down. (01:21:05)
Continuity mistake: When Sam and Harry get out of their separate taxis near the start, there is a huge amount of rope on the harbour. Camera cuts and it's now disappeared and is not trailing from the boat. (00:07:50)
Continuity mistake: In the Super Trouper scene, when we see Donna, Tanya and Rosie raising their left hands, we see that Rosie is just behind Donna's sleeve. However, in the next shot, in which we only see Donna, we can't see Rosie or her shadow at all.
Continuity mistake: When Donna is dancing to "Dancing Queen," her hair is all messed up. But in the next shot, her hair is not messed up.
Continuity mistake: During Dancing Queen Donna is jumping on the bed. Between the camera angles, her position keeps changing.
Continuity mistake: At the end of Voulez Vous, the people pick up Sophie. In one shot her hair falls down to her shoulders, camera cuts and it falls back down again.
Continuity mistake: When Tanya is singing "Does Your Mother Know", she walks away from the bar and goes out to the beach to join the crowd of men who are dancing. As she's walking, there's a side shot of her, and in the background you see an extra with a towel around her waist wearing red wayfarer sunglasses. But then there's a face-on shot of Tanya, and the extra's sunglasses have disappeared. (01:10:30)
Continuity mistake: When Sky is on the beach with Sophie, his arms are constantly changing place depending on the shot. This is not due to dancing.
Continuity mistake: When singing Mama Mia the first time, the position of the mum's arms are constantly changing between shots.
Continuity mistake: When Tanya first arrives, at the end of the song, her arms constantly change place between shots.
Answer: Sophie finds Donna's diary from 1979, being the year Sophie was conceived meaning she was born in 1980 (as the diary entries are from July/August). So the movie is set in 2000.