Great point Slamfire. It's important to realize that some molds do drop oversize bullets by design. If a guy had a mold like that and didn't run them through a sizer, instead just tumble-lubing them with liquid Alox, that could definitely cause problems.
My .30 cal molds both drop .311 diameter bullets with my alloy, and I size them with a .310 sizing die. There are oversize .30 cal molds that drop bullets at .313 - .314, usually intended for .303 British, but sometimes they just come from the manufacturer a little oversized due to poor quality control.
Thanks for pointing that out Slamfire!