On 2 March 2023, an Airbus A330-900 level at FL390 encountered en-route turbulence unanticipated by the two First Officers occupying the pilot seats whilst the Captain was in the crew rest area. All three pilots had only recently qualified on type. A failure to recognise the risk of significant convective turbulence in advance led to the seat belt signs being off with no prior warning given to either passengers or cabin crew which resulted in 6 severe and 16 minor injuries all requiring hospital treatment. Multiple safety actions were implemented by the aircraft operator to reduce the risk of recurrence.