I'm sure many of us have been to at least one wedding. Have you seen anything at other weddings that you thought "I'll have to remember to not do that for my wedding..."
For me, one was a relative who spent the week before her wedding running around doing little errands and buying some of the decor. I realized I wanted to have that all done.
My SIL got her hair dyed the week of the wedding and it came out too dark so she had to get it re-done a day or two before the wedding.
And, another friend didn't change his seating chart after some people backed out and he invited others (we were a close group of 7 families and he only invited those his age at first and then the parents later) so the parents didn't know where to sit and our table became extra crowded while other tables ended up being almost empty. So, I learned to revise the seating chart when people backed out!
Those are all the lessons I can think of at the moment!