UA 935 (LHR-LAX) made a U-turn just south of Iceland and is returning to London, according to Flightradar24:
It's interesting because it's not squawking 7700, so it's no emergency due to mechanical problems.
What other issues could cause a plane to turn back after already going that far, that isn't a "real" emergency? Medical issues? Passenger on a no-fly list? That's certainly a long flight for those passengers, to be going "nowhere"