Mine was this. A guy called my sim up and invited her to the romance festival, and my sim agrees to go. Now, this guy is married, and also another sim that I also play, but in another household. So I thought it would be a good idea to include his wife with this group. Everyone goes and drinks the tea that makes everyone flirty. Almost immediately some other woman walks up to the married guy and flirts with him, and he flirts right back. All this is happening right in front of his new wife (still newlyweds). She gets extremely angry and walks off.
My duh moment is never to invite another married couple to one of those things unless I can actually control everyone.