Certain things happen consistently on all of my trips. Long or short, domestic or international, winter or summer. I always try (and usually succeed) to travel only with carry-on luggage, at least on the way out. The one time in recent memory when I decided to check my suitcase, I arrived in Madrid 3 days before my luggage. But that's Murphy's Law, not necessarily a Travel Law.
The Travel Law dictates that no matter how good you are at packing, the second you open the suitcase at your destination, it can never, EVER, be repacked as efficiently again. What fit the first time will no longer fit. What folded just right the first time will never do so again. The shoes you carefully nestled against the sides of the suitcase appear to increase two or three sizes. Your belongings expanded. Or your suitcase shrank.
I cannot count the number of times I said to myself there was no need to take a folding duffel bag, and had to buy one in whatever country I end up. If I'm heading home I am likely to check even my carry-on - if it is delayed, no big deal. So there is really no excuse for not packing the stupid duffel. Instead, I stubbornly think the carry-on should fit the exact same things it fit before the trip started, dammit.
And here I am again. Swearing at the suitcase. Sitting on it. Wondering if I can buy a folding duffel bag at the airport.