diff --git a/src/grid/grid_geometry_cartesian_3d.f90 b/src/grid/grid_geometry_cartesian_3d.f90
index 6c0ea0ef..2d1ccd6b 100644
--- a/src/grid/grid_geometry_cartesian_3d.f90
+++ b/src/grid/grid_geometry_cartesian_3d.f90
@@ -250,10 +250,12 @@ subroutine place_in_cell(p)
        p%killed = .true.
     else
        p%in_cell = .true.
+       ! Only adjust the wall position for photons that were successfully
+       ! placed in a cell - for killed photons the cell indices are invalid
+       ! and adjust_wall would index the wall arrays out of bounds.
+       call adjust_wall(p)
     end if
 
-    call adjust_wall(p)
-
   end subroutine place_in_cell
 
   logical function escaped_photon(p)
diff --git a/src/grid/grid_geometry_cylindrical_3d.f90 b/src/grid/grid_geometry_cylindrical_3d.f90
index 0dd5ae33..1811c84a 100644
--- a/src/grid/grid_geometry_cylindrical_3d.f90
+++ b/src/grid/grid_geometry_cylindrical_3d.f90
@@ -358,10 +358,12 @@ subroutine place_in_cell(p)
        p%killed = .true.
     else
        p%in_cell = .true.
+       ! Only adjust the wall position for photons that were successfully
+       ! placed in a cell - for killed photons the cell indices are invalid
+       ! and adjust_wall would index the wall arrays out of bounds.
+       call adjust_wall(p)
     end if
 
-    call adjust_wall(p)
-
   end subroutine place_in_cell
 
   logical function escaped_photon(p)
diff --git a/src/grid/grid_geometry_spherical_3d.f90 b/src/grid/grid_geometry_spherical_3d.f90
index ec99307d..1ad35c06 100644
--- a/src/grid/grid_geometry_spherical_3d.f90
+++ b/src/grid/grid_geometry_spherical_3d.f90
@@ -476,10 +476,12 @@ subroutine place_in_cell(p)
        p%killed = .true.
     else
        p%in_cell = .true.
+       ! Only adjust the wall position for photons that were successfully
+       ! placed in a cell - for killed photons the cell indices are invalid
+       ! and adjust_wall would index the wall arrays out of bounds.
+       call adjust_wall(p)
     end if
 
-    call adjust_wall(p)
-
   end subroutine place_in_cell
 
   logical function escaped_photon(p)
diff --git a/src/grid/grid_geometry_voronoi.f90 b/src/grid/grid_geometry_voronoi.f90
index 6af303fc..03dd3406 100644
--- a/src/grid/grid_geometry_voronoi.f90
+++ b/src/grid/grid_geometry_voronoi.f90
@@ -144,7 +144,7 @@ subroutine setup_grid_geometry(group)
        ! Specifically:
        ! - sparse_idx(is) + 1 because sparse_idx is an array of C-style indices,
        ! - sparse_neighs(...) + 1 because the neighbour indices are C-style.
-       geo%cells(ic)%neighbors(1:n_neighbors) = sparse_neighs(sparse_idx(is) + 1:sparse_idx(is + 1) + 1) + 1
+       geo%cells(ic)%neighbors(1:n_neighbors) = sparse_neighs(sparse_idx(is) + 1:sparse_idx(is + 1)) + 1
        is = is + 1
     end do
 
diff --git a/src/grid/grid_propagate_3d.f90 b/src/grid/grid_propagate_3d.f90
index 41549b4b..12794d5d 100644
--- a/src/grid/grid_propagate_3d.f90
+++ b/src/grid/grid_propagate_3d.f90
@@ -71,6 +71,11 @@ subroutine grid_integrate(p,tau_required,tau_achieved)
 
     if(.not.p%in_cell) call error("grid_integrate", "photon has not been placed in a cell")
 
+    ! A photon emitted exactly on the outer edge of the grid moving outwards
+    ! is placed by adjust_wall in the cell just outside the grid, so it
+    ! escapes immediately without crossing any walls.
+    if(escaped(p)) return
+
     if(allocated(n_photons)) then
        if(last_photon_id(p%icell%ic).ne.p%id) then
           n_photons(p%icell%ic) = n_photons(p%icell%ic) + 1
@@ -259,6 +264,11 @@ subroutine grid_integrate_noenergy(p,tau_required,tau_achieved)
 
     if(.not.p%in_cell) call error("grid_integrate_noenergy", "photon has not been placed in a cell")
 
+    ! A photon emitted exactly on the outer edge of the grid moving outwards
+    ! is placed by adjust_wall in the cell just outside the grid, so it
+    ! escapes immediately without crossing any walls.
+    if(escaped(p)) return
+
     if(tau_required==0._dp) return
 
     ! Check what the distance to the nearest source is
@@ -388,6 +398,14 @@ subroutine grid_escape_tau(p_orig,tmax,tau,killed)
 
     if(.not.p%in_cell) call error("grid_escape_tau", "photon has not been placed in a cell")
 
+    ! A photon emitted exactly on the outer edge of the grid moving outwards
+    ! is placed by adjust_wall in the cell just outside the grid, so it
+    ! escapes immediately without crossing any walls.
+    if(escaped(p)) then
+       tau = 0._dp
+       return
+    end if
+
     ! Check what the distance to the nearest source is
 
     call find_nearest_source(p%r, p%v, t_source, source_id)
@@ -485,6 +503,14 @@ subroutine grid_escape_column_density(p_orig,tmax,column_density,killed)
 
     if(.not.p%in_cell) call error("grid_escape_column_density", "photon has not been placed in a cell")
 
+    ! A photon emitted exactly on the outer edge of the grid moving outwards
+    ! is placed by adjust_wall in the cell just outside the grid, so it
+    ! escapes immediately without crossing any walls.
+    if(escaped(p)) then
+       column_density = 0._dp
+       return
+    end if
+
     ! Check what the distance to the nearest source is
 
     call find_nearest_source(p%r, p%v, t_source, source_id)