What event frightened Jonas about a year ago?

About a year ago, Jonas was frightened by an Incident in which he witnessed a plane flying overhead, which was unusual in his controlled community. The aircraft's presence caused panic, as it was a breach of the community's strict rules and highlighted the outside world's unpredictability. This event made Jonas aware of the limitations and dangers of his sheltered life, deepening his sense of unease about the society he lived in.