The Principle of Double Effect is often introduced in terms of weighty cases of killing, like bombing military installations or redirecting trolleys. But the importance of the distinction between intended and unintended but foreseen harms can be seen even more clearly in everyday cases.
Yesterday, my wife went grocery shopping, while I was home with some of the kids. My son asked to be taken for a bike ride. The thought flashed into my head: “If I go, I probably won’t be home when my wife comes back with the groceries, and hence I won’t be able to help with unloading them.” There are three possible attitudes I could have with respect to this observation:
I shouldn’t take my son for a bike ride now.
Not being able to help my wife is an unfortunate side-effect of taking my son for a bike ride.
Being able to get out of helping my wife is a reason to take my son for a bike ride.
In cases (2) and (3), the foreseen effects are the same. There are no deontic issues (I didn’t promise my wife to be home). But clearly if I take attitude (3), and hence intend not to be there when my wife comes back, I am being a bad husband, while if I go for (1) or (2), my behavior is defensible. (In fact, I never got around to taking my son for the bike ride.)