Germany and Supplies
Unfortunately for the Poles, the Germans won too quickly for this to be a factor, but the Germans did have major supply problems toward the end of the invasion of Poland. As I recall it, they were very close to out of bombs. I haven't heard similar reports on other types of munitions, but I wouldn't be surprised if they were running low there too. The Germany military of September 1939 was very much an "all in the showroom window" military.
The Germans had a little over 5 years to build specialized military industry, to train an army, to build tanks, planes, artillery, ships, etc. They were constantly on the edge of running out of the hard currency they needed to import raw materials they needed for the buildup. They couldn't do everything they needed to get ready to fight a war. In the lead-up to September 1939 they built a lot of big-ticket items--tanks, planes, artillery. They didn't make enough ammo or spare parts to keep those big ticket items functional in a long war. That's a major part of the reason the Germans didn't try an attack in the west in November 1939, when Hitler wanted them to.
The Germans sort of learned their lesson, at least for a while. They spent the seven and a half months between the wind down of the conquest of Poland and the invasion of France building up enough ammo and spare parts to take on a major power. Then they managed to forget the lessons of Poland when they invaded the Soviet Union. They produced what they considered adequate supplies for that war before they invaded, and then started retooling to give priority to naval and air power for the war they intended to fight after they crushed the Soviet Union--the one against the British and US. Oops.