Not too terribly long ago I saw something similar and another similar follow-on post with the rule: "One powder at the bench at a time".  Not sure if or how often I violated that rule before but I adopted it right then.    

I've caught myself in errors before, fortunately before they cost me.   Keeps me from being cocky and realizing the more checks I use the better.    

I like the note about the scales.    I have not done that before but worth noting.

My late friend was very recoil sensitive and loaded LOTS of reduced loads, using fast-burning powders.  He liked to cast bullets and load his bullets into military-size cases. He was very strict about only having one can of powder at a time on the bench and in his powder measure.   

His method, loading in batches of 40 or 50, was to use a powder measure, but drop the load into the pan of his powder scale.  He then trickled in the last few granules of powder to bring the load up to desired weight.  

When he had weighed powder charges in the entire batch of primed cases, he would check the level of the powder in each case with a tiny flashlight.

And then he would check the powder level again with a wood dowel marked with the proper caliber, powder & load.  He had a line on each dowel.  He had a collection of what must have been nearly a hundred dowels, each carefully made & marked.   He called these his "no screw-ups" gauges.  Only after the dowel check would he seat bullets.

When my friend passed away (heart attack), his widow asked me to get rid of all the reloaded ammunition.  That was 2002.  My grandson & I have been shooting up the stuff for almost 13 years and we are nowhere near the end of the stash.