Cluster node may enter in unresolved conflict state when there are two inserts, with same primary key, into a table which has foreign key constraint for a parent table. These inserts must be issued in separate cluster nodes, and there has to be simultaneous writes (updates or deletes), for the referenced parent row.
As a result of such scenario, replication applier thread may end in unresolved conflict state, And error log will be filled by messages of type:

"WSREP: BF lock wait long"

followed by InnoDB monitor outputs