On every leg of my itinerary (below), I checked in one bag (well under the weight limit).
I was not charged a baggage fee on the 1st leg (DTW-LAX on AA). And of course I wasn't charged a fee on the 2nd or 3rd legs (SFO-SIN, SIN-IAD, both on United).
However, I was charged a $25 baggage for the 4th and final leg (IAD-DTW, US Airways). Why? Might this possibly have been a mistake and should I not have paid?