Last night, I heard my wife and our son fighting. I went upstairs and asked what was happening. My wife starts explaining about how our daughter snuck her kindle when she was not supposed to have it. OK, but what does that have to do with why she was fighting with our son? Oh, because she decided that it was the perfect time to pick a fight with him because our daughter must be copying his behavior! I tried several times to explain to her that it was not a good idea to start widening the fight. She just kept repeating that our daughter was influenced by his bad behavior and acted like I was denying that that was a factor. I responded by saying that whether or not that was a factor was not the point. The point was that if she is having a fight with one of them she should not bring the other one into the fight. It only gets everyone, including her, more angry and makes it harder for her to make any progress with either one of them.
Finally, hours later, she apologized.
My wife has confused "picking your battles" (not fighting over everything) with picking fights (finding things to fight over.)