This error does occur when radiation history tracking fails i.e. in the non-overlapping part of GGI interface. Previous versions before 18.0 only accidentally worked and those tracks were actually being lost. A sanity check was added in 18.0, which results in this error message when a track hits a non-overlap face. Fixed in 19.3. Full error message: +——————————————————————–+ | ERROR #001100279 has occurred in subroutine ErrAction. | | Message: | | Stopped in routine WALK | | | | | | | | | | | +——————————————————————–+