Shouldn't be a problem at all, so long as the powder from both containers is in good shape. Probably not ideal, but yeah, I'll admit I do this quite often as well.
There ARE differences from lot to lot that the manufacturer has little control over. While it is an specific formulation, you have to remember that powder is made from organic material (usually wood pulp) that varies naturally. While the lab has some ability to modify and correct these variables, there's a certain percentage that they just have no control over. That, in large part, is where these lot to lot variations comes from.
As far as 4xforfun's mention of the weight variation, I'd be inclined to attribute that to moisture content. Powder is higly hygroscopic and can (and will) take on or bleed off moisture depending on how it's stored. Had his buddy gone ahead and loaded the rounds without adjusting the measure, I'd guess that his load would have probably shown little or no variation from the previous load (albeit a differnt charge weight) would have shown. My predecessor, Martin Hull taught me this many years ago when I first started working in the lab. He'd check charge weights when setting up a measure, but only to verify that it was roughly in the ballpark. Beyond that, he set it strictly by the settings recorded for the last loading session. The measure was a Lyman, modified by Seely Masker many years earlier and used a micrometer barrel to set the powder chamber. Very repeatable, and very precise. At any rate, it was loading by volume, rather than strictly by weight, and always gave consistent results.
