If a ForkJoinNode has an ObjectFlow in its incoming edges, it must have an ObjectFlow in its outgoing edges and vice versa. The same applies for ControlFlows.
UML constraint:
All information entering a ForkJoinNode cannot be lost and all information going out of a ForkJoinNode cannot be created.
When you get an R1250 warning on a ForkJoinNode, check the consistency of flows going in and out of the ForkJoinNode.