This happens on Stock 4.9. When using SpawnActor with the Template parameter set to an actor so that it gets cloned, USCS_Node::ExecuteNodeOnActor eventually attempts to attach the root component unto itself leaving masses of warnings in the editor logs.
The following project reproduces the issue. Press play in the provided level, then press “F6” to trigger cloning of an actor (F6 handled in the level blueprint). Warning will be logged, and user notified when stopping play.
Attached repro project : link text