The parents come back home, see the living-room is in a terrible mess.
Do they ask their little ones "What have you done?" or "What have you been doing?"
What is understood in each of the questions?
I call this a tricky case because the usual criterion to distinguish between Present Perfect Simple and Present Perfect Continuous, the focus on the result of the action for the simple, and on the action itself for the continuous is a bit difficult to use here, isn't it?
These are mostly idiomatic by now. It's rarely a question about actual activity, more of accusation, complaint, exclamation.
"What have you done?" is a rhetoric question, "Look what you've done", not requesting information, but more of apology. It's a complaint, often expression of grief, like "How could you?". The focus is on the current state, and effects of the activity, possibly sudden.
"What have you been doing?" is more of exclamation of outrage or wonder - what kind of process could have led to this. The effects are either a result of continued activity or continued negligence - not something sudden, but something that took time, a lengthy destructive process. It's often associated with anger - because the results weren't an unexpected accident but an effect of continued, lengthy failure. Also, the focus is on the process that led to these effects: "How did this happen?"
