L'indice n'appartient pas à la sélection...

Private Sub ComboBox1_Change()
    Set f = Sheets("Feuil2")
Set mondico = CreateObject("Scripting.Dictionary")
  For Each c In Range(f.[A2], f.[A65000].End(xlUp))
    If c = Me.ComboBox1 Then mondico(c.Offset(, 1).Value) = c.Offset(, 1).Value
  Next c
  
  If mondico.Count > 0 Then
    temp = mondico.items
    Call Tri(temp, LBound(temp), UBound(temp))
    Me.ComboBox2.List = temp
    Me.ComboBox2.ListIndex = -1
    'Me.ComboBox3.ListIndex = -1
  End If
End Sub