They were pretending to target foreigners for surveillance and in doing so got "incidental" contact with Americans. The rub is that the American contacts were really the targets. Did they tell these foreigners who to contact? Was there a back-channel to get this to happen?
Normally, Americans names are masked to protect American civil liberties and in only very strict cases should these names be unmasked. The Obama admin unmasked Americans they suspected to be working with Trump campaign to start an investigation into the Trump campaign. Planned? Or incidental?