3 or 4?
I remember one group shot I took, only 6 people in it. The sequence was like this.
1 - Mother of the bride blinked
2 - father of the groom licking his lips
3 - groom blinked
4 - best man looked to the right across the group, like he was checking what everyone is doing
5 - Mother of the bride blinked again
6 - father of the bride made a weird shape with his mouth
7 - One of the bridesmaid looking at somewhere else
at which point i am crapping myself....where is this shot I thought i saw in the LCD that i thought it passed the test ?!
8 - something else wrong
etc etc
Other little things such as:-
Mother of the bride/groom wearing hats, it creates a shadow under their face so it will ALWAYS be under exposed. They are reluctant to take it off as its part of their outfit, some even continue to wear it throughout the evening do.
Some men wearing reaction lenses or sunglasses. They forget they have them on and remember to ask them to take it off.
Wind making hair flying across their faces
Big groups tend to stand in a horseshoe shape instead of a straight line........it is a weird human nature thing and I don't know how it happens, the people on the edges just unconsciously step forward, the more to the edge they are, they step further forward. So make sure they are, unless you want to shoot in F/11. (noticed how that shot above they are lined up straight across)