Inspired by the dollar dance thread.
Like most here, I try to follow etiquette because I would never want to unintentionally offend someone I love or care about. That said, I also try not to judge mistakes from others because I assume they are doing their best and don't know any better--after all, I'm the girl who dictated attire on her wedding invitations not six months ago, something that makes me cringe with embarrassment now. There are even a few rules that I think are sort of stupid (like the watch thing). However, there are some things that bother me so much, I would have to take action.
1.) Dollar Dances- I would get up and walk out of any wedding with one of these, probably even a family wedding (though for very close family members I would probably claim a headache or something).
2.) Not inviting my husband when I am invited-Before I met DH, I did not understand this one, but I get it now, and I would not hesitate to decline an invitation that did not include him (women only events obviously excluded). I would not explain why unprompted but would not hesitate if asked.
There may be more, but that's off the top of my head.