One instance that comes to mind was a trip from ORD-BOS with my then-gf (now wife) to visit some friends. She had the window, I had the middle, chivalrous fellow that I am. Once we reached cruising altitude, I reclined my seat partially (I rarely recline all the way, and I ALWAYS do so slowly after sneaking a peek behind me). The man sitting behind me didn't say anything, but who I assume to have been his wife in the aisle started muttering under her breath something about rude passengers reclining their seats and encroaching in their space.

I turned around and asked if everything was ok, to which she replied that everything was fine. Upon turning around, she started muttering under her breath yet again, but this time with more colorful language. Again, I turned around, this time asking if there was a problem and that, if so, she should let me know. I was met with a civil, if not polite, rebuff once again. A third time, she muttered under her breath ramblings along the same lines. Finally, I turned around and told her in no uncertain terms that if she has a problem, to be forthright with me rather than doing this passive-aggressive muttering, and that her husband - who hadn't said a word the entire time - was the one directly behind me anyway and didn't seem to mind, so what was her problem.

The rest of the flight was extremely peaceful (for me and my gf, at least).