Revealing mistake: After Sophie faints at her bachelorette party, during the next scene it cuts back and forth between shots taking place in two rooms - Donna walking into Tanya and Rosie's room, then Sophie walking into Ali and Lisa's room, and both rooms have the very same cracks and stains on the walls revealing it was filmed in the same room, but with different furniture.
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.
Revealing mistake: During the "Lay Your Love" song, when Sophie and her boyfriend are in one of the corners of the beach, sneaker footprints are seen. Given that the tide is constantly wiping off their bare footprints, the tracks most surely belong to a crew member. (00:44:20)
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.