Say I make a general statement that something is bad, then I want to "accuse" someone of having done that.
As per the title, what is more appropriate ? "Did you just do that" vs "Did you do just that" ?
Both your sentences get into idioms.
"Did you just do that?" Usually it would be taken to mean, "Did you do that very recently?" Like we say, "I just got off the plane", meaning, I got off the plane within the last few minutes, as opposed to hours ago. It could also mean, "Did you do that and nothing else?" Like, did you ONLY push the green button or did you push the green button and also other buttons.
"Did you do just that?" would usually be taken to mean, "Did you do that and nothing else?", as above. It could also mean, "Did you do exactly that?" Like you say you put the card in the slot with your left hand. Are you sure it was your left hand and not your right hand?
If I wanted to ask someone whether they had done something or not, I would most likely ask simply, "Did you do that?" and not add the word "just" to the sentence.
Did you just do that? Or did you do other things as well?
Did you do just that? Exactly that? Or did you do something slightly different?