Visible crew/equipment: While Sophie's with Lisa and Ali at the dock, after they admire Sophie's engagement ring Lisa bends down to pick up her handbag, and there's a rather large bulge at her lower back under her top - presumably the mic pack, which wasn't there when Lisa lifted Sophie and spun her around.
Continuity mistake: While Ali and Lisa are trying on the dresses for Sophie's wedding, after Sky leaves the room with his props, when Ali and Lisa stand up only one side of Ali's dress is over her shoulders in the shot facing Sophie, but in the shot facing Ali and Lisa both sides are over Ali's shoulders properly.
Continuity mistake: When Rosie and Donna are trying to get Tanya's boots off, Tanya is lying flat on her back. The camera does an instant cut and she is now propped up.
Revealing mistake: Towards the end of the Money Money Money song there is a scene where Donna, Rosie and Tanya are sitting in the Captain's deck. If you look at Rosie you will see she is looking through binoculars. Look closely and you'll see that the lens caps are still on.
Visible crew/equipment: Near the end, when Rosie is asking Bill to Take A Chance On Me, she falls and is hanging from the roof. In one shot when she has her glasses, a light screen can be seen reflected in them.
Continuity mistake: When they are singing Does Your Mother Know You're Out, a man is seen getting out of water and is soaking wet. However, a few shots later he is completely dry.
Other mistake: Donna sings The Winner Takes It All and it is nowhere near sunset. During the song she makes her way up to the church. But when she arrives, there is a good sunset.
Continuity mistake: During "The Winner Takes It All," you first see the cliff with the church and the sun very low on the horizon. Later during the scene, the sun is much further to the right and higher in the sky (the sun glare on the water is bright white). The scene was apparently filmed in the morning (and not in the evening) and we see a sunrise and not a sunset.
Visible crew/equipment: When Sam is proposing to Donna, you can see a screen reflected in Rosie's glasses.
Continuity mistake: When Sam tells Sophie that they could do a test to see who her father is, Sophie raises her hands to her face twice.
Continuity mistake: When Donna is fixing the crack in the tiles, the tip of the filler-gun breaks off. She stands up, and a bit later waves the filler gun, with the tip back on. The pistol has been out of sight (her hands down) during the scene, but she didn't look down once, and we see that she is carrying a bowl in the other hand, so she would not have had a chance to put the tip back on again.
Continuity mistake: At the chapel, Sam has just proposed to Donna. She says she's not a bigamist. Sam says neither is he and that he has loved Donna for the past 21 years. To his right stands Sophie, who after hearing Sam say this, is all choked up and looks like she's about to burst into tears of joy. Immediately after you see this expression we see Donna very briefly (one or two seconds). When the shot goes back onto Sam and Sophie, Sophie has a huge grin on her face. Nothing is said by Donna or Sam in this extremely brief period of time that would justify her instantaneous change of expression.
Continuity mistake: During the wedding of Donna and Sam, the rings on Donna's fingers are disappearing and reappearing.
Other mistake: At the end of them singing "Money, Money, Money", when Donna is screwing the falling window shutter back, there is no drill bit on the drill, making it not possible for her to put the shutter back up.
Continuity mistake: When Sam and Harry's taxis arrive at the pier, the sunlight casting on the pavement changes dramatically between shots.
Visible crew/equipment: When Donna's friends are singing to Dancing Queen you can see a crew member reflected on the hairdryer.
Visible crew/equipment: At the end of a song near the end at the wedding reception, the stage lights and a light screen can be seen in the champagne glasses of the guests.
Continuity mistake: In the scene where Donna is crying and having her nose wiped her nose is very red. As soon as she leaves the building and crawls on the roof her nose is just fine.
Factual error: Sam and Donna would not be able to marry. There are things which need to be done before it is legal for two people to marry and it is therefore not possible to marry minutes after proposing.
Visible crew/equipment: When Donna and her friends are singing "Super Trooper" at the bachelor party, you can see a crew member reflected on the colorful door.
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.