Corrected entry: In one of the first scenes where the characters wake up, there is a shot where a breakfast cereal's package is seen in the lower left corner of the screen. The brand image of that package is reversed.
Corrected entry: This is a strong contender for the mistake earliest into a movie: When Charles' alarm clock rings at 9.00 the hour hand hasn't reached the set time yet (about 9.30). This happens 7 seconds after the movie actually starts, and it happens again when he gets up for the second wedding. (00:00:35 - 00:33:15)
Correction: I've had cheap alarm clocks with exactly this behaviour where the alarm goes off when the hour hand gets close to the alarm hand. That it happens twice supports the fact that it's a "feature" of the clock rather than a movie mistake. The trick to using such clocks is to adjust the time to your desired waking time and then adjust the alarm hand until the alarms sounds, then adjust the time back. In all probability the alarm clock was just set to go off anyway, rather than having the sound dubbed afterwards, so that's just how it worked.
Corrected entry: After the first wedding, the group is making their way across the lawn towards the receiving line. One shot shows them from behind, and then it cuts to view them from the front. In the scene from behind, David is missing. Matthew can be seen with his arm around Scarlett, but when they switch to the front view, David's arm is around Scarlett, and Matthew is walking ahead of them, with Fiona and Gareth.
Correction: The front view is taken at a totally different place, and there was been plenty of time for them to regroup (although the dialog that belongs to the later shot already starts in the previous shot, but this is more of an artistic tool than a mistake).
Corrected entry: In a scene after the funeral, all the mourners are standing on a dock facing the water - if you look at the water it has little waves in it that do not move (it's a still shot of water).
Correction: Correction - the ripples are in mud, because the tide's gone out.
Corrected entry: When Charles thanks Matthew for being his best man, Matthew apologizes for being late and says "9:45, 45 minutes till I do". This means that Charles is getting married at 10:30. After Fiona wipes lipstick off Charles's cheek, we see a close up of the church clock which reads 10:40. We then see people arriving before it cuts to the priest and Charles inside an empty church. Even allowing for the fact that Charles is always late for weddings and tradition states that the bride should be 20 min. late, surely there would be some people inside the church waiting for the ceremony to begin at 10:30.
Correction: First, the church clock could simply be wrong. Second, Charles and the priest (after Charles has seen Carrie and is saying 'bugger, bugger, bugger') are not in the church proper, where the wedding ceremony will be performed, but in some sort of side chapel or vestry area, where the groom awaits his fate.
Correction: This is because we see the image of the actor and the cereal box reflected in a mirror.