I do the one-at-time thing, too. I also reread the label every time, before pouring anything.
I just started experimenting with Diafine. If I get the two parts mixed up, I am screwed. If ANY amount of Part-B gets into Part-A it will be ruined. Therefore, I need to double down on paying attention to what I pour, where and when or else I risk trashing $50 worth of chemistry.
It might seem silly at first but, after a while, it becomes a habit.