Someone wore a white dress to my wedding but it was a kind of an off-white macrame/crochet style, and it was very clearly not a wedding dress. Sort of similar to this one in essence.
Even though I spoke to her during the wedding, I had so much on my mind that the fact it was white went completely over my head. It didn't click until I was looking at photos later, and even then, it made me laugh more than anything! It helped that we had a beach wedding and her dress was very beach-y, and that she was a distant enough relative of my husband's (and nice enough person) that while it was a weird choice, it seemed innocent enough.
I read somewhere that guests wearing white - apart from being "disrespectful" - is bad because it draws attention away from the bride in photos, but there were so many colours all over the place that it really didn't; same goes for two men who had white/cream jackets.
That said, I did have to rope my mum in to make sure my aunt didn't show up wearing white, which she was threatening to do. She is a very glamorous woman and knowing her it definitely wouldn't have been a quiet beach dress like the other guest, but thankfully my mum managed to talk her down in time...