At the beginning of the movie when Julianne (Julia Roberts) and Kimmy (Cameron Diaz) are in the bridal shop and Julianne is trying on her lavendar gown for the wedding, the straps are solid satin. However, when Julianne is at their wedding at the end of the movie, her straps are all "sequiny", not the solid satin they were at the beginning.
It's possible that they changed the dress between her trying it on and actually wearing it. They were working on it in that scene, Jules even ripped one of the seams.