In the last flashback of the day after Niles and Daphne's first night together, it says the year is 2002, but the episode "Daphne Returns" happened in 2001.
In the flashback of 1996, Niles says Maris is upset at Frasier but Niles and Maris separated near the start of season 3, which was 1995.
Niles and Daphne had their honeymoon in 2002, not 2003.
Daphne asked Niles how he knew how to remove the superglue with acetone. She was the one who actually told him in "An Affair to Forget" when his own hand got stuck on that ship that he and Martin were putting together.
This episode is not included in the Final Season DVD boxed set.
July 15, 2004, the date the wedding should have been, was not a Saturday.
When Niles drops the ring, Eddie is eating from a plate. On a pulled-back shot, Eddie has licked the plate clean; but on the close-up of Eddie, the plate still has food on it. On the next away shot, Eddie is still licking a clean plate.
Eddie's birthday is May 15 in this episode, but in an earlier episode his birthday is said to be on May 7.
When Niles drops the ring, it can be seen bouncing out of camera range and not into the pate.
While gathered together on Frasier's balcony, Martin, Frasier, and Niles are talking. Frasier mentions "It's just an hour by plane" (between San Francisco and Seattle). Most commercial flights run in the two hour to two and a half hour range between those two cities.
In "Don't Go Breaking My Heart (3)" in the flash-forward to Niles' and Daphne's second child's birth, Niles is holding a girl. In this episode, Daphne gave birth to a boy.