Continuity mistake: During Does Your Mother Know one of the girls in the background has red sunglasses which she wears in one shot and doesn't in the next - it keeps changing.
Continuity mistake: When Lay All Your Love on Me starts, right before Sophie's first singing lines, she has some hair in front of her face. But, when it switches to a shot in front of her as she continues to sing, she immediately has no hair in front of her face. She didn't have the time to move it out of the way in-between shots.
Continuity mistake: During the song "The winner takes it all" at one point 4 people are walking up the mountain in the background, 2 in white and 2 in mint green. The next shot they are gone.
Continuity mistake: Just as Donna starts singing the last of 'Slipping Through My Fingers' Sophie is put on a donkey to be taken up to the hall for her wedding. Donna and Sam are left alone and Donna sings "The Winner Takes it All' but when she gets up to the hall the donkey is nowhere to be seen.
Continuity mistake: When Sophie is about to send the letters, the first one to Sam has wrinkles on the left, but is smooth in the following angle.
Other mistake: When Sophie swims to the boat with her three possible fathers on it, when she takes off her clothes to reveal her swimsuit, she takes a very short time to take them off, too fast for them to actually be off.
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.