Sure I'll bite
I was commuting home from GSO to DAB via ATL with Delta...got in a bit of a tussle with an FA. I was a fairly new FO, had not commuted much before. Leg from GSO to ATL was uneventful. Got on a mainline Delta MD88 heading to Daytona, the flight was full with all the standby listers, but I managed to get a seat. The gate agent told me as I walked down the jetway to ask the gentleman in my seat to move...odd, but OK. I was back in row 35 or thereabouts, and found my seat occupied as expected, and I showed the gentleman my seat assignment. He explained that he was a first class passenger, but wanted to sit in the back with his wife, and that I could take his seat. I was in the back of the airplane at this point, so I asked the closest FA if it was OK for me to sit in the guy's first class seat. He said it was fine, so I waited for the rest of the pax to board, then walked back up front to first class, where the guy's seat was open next to the window, with the aisle seat occupied. The lady in the aisle seat got up, I sat down. Just sat down when the lead FA came storming back and said "Where is your assigned seat?"
I explained to her what just happened. She interrupted me and said "No, sir, you need to go back to your assigned seat." She was pretty rude about it. Of course all of first class was being entertained by this at this point. Being a pretty patient guy, and not wanting to make a scene, I apologized to the aisle seater for making her get up again, and walked back to the back again. Lead FA followed me this time, and the first class guy explained to her the same thing I had just told her, to the laughs of the pax all around us. She just said "OK!", and stormed back up front, not saying another word to me. I walked baaack up front again, promised the aisle seater this was the last time, and sat back down in first class. Got to Daytona on time and all was well! Amazing what a little lack of communication can do. She apologized to me at the end of the flight, but that was not the most pleasant commuting experience!