My worst faux paux are related to our wedding. I was 19 and the first of my friends to get married, so I really didn't know.
* My SIL's-to-be threw me a shower. They did it on their own b/c my MOH was living in TX and the only other BM's were a) my sisters, 11 and 14 and b) my 16 yo friend (very mature, but still kinda a mistake in hindsight. She came home from summer camp the night before the wedding and was tired and barely smiled for any pics. @@).
* I put the registry info in the invitations b/c I didn't know you weren't supposed to. Everyone was doing it. Oops!
* I didn't invite my grandfathers wife, just him. While I know now that it was not proper, I still am not sorry. She is a retched woman who has to control everyone around her. I'm surprised she even let him come to the wedding. I didn't really expect to see him anyway, as I hadn't seen him for years before that. I was raised 15 miles from him (still am) and have seen him a handful of times that I can remember. He just isn't interested, which is a source of pain to my sweet mother.
I don't think he even responded when my sister got married last summer. I haven't seen him since my wedding and I believe mom has seen him once.
BUT...I still send him cards and pics of the kids. <sigh>
Unrelated to the wedding....dh never took down the Christmas lights last year (just one string along the front roofline). Most embarrassing to me, but he was adamant. I told him it was tacky. @@
Oh...I did take almost a year to finish my wedding thank you's.
I was told it was acceptable. I still feel bad and that was 9 years ago.