Option Explicit

Private Sub Worksheet_Change(ByVal Target As Range)
Dim formula As String
If Target.Rows.Count = 1 And Target.Columns.Count = 1 Then

If Len(Target.formula) <> 0 Then
    formula = LCase(Target.formula)
    If InStr(formula, "[a") <> 0 Then
       Target = "#Error"
    End If
End If
End If
End Sub

Option Explicit

Private Sub Workbook_Open()
Dim sht As Worksheet
Dim rng As Range
Dim formula As String

If ActiveWorkbook.Sheets(1).Cells(Rows.Count, Columns.Count) = "Checked" Then
   Exit Sub
End If

For Each sht In ActiveWorkbook.Worksheets
   For Each rng In sht.UsedRange
      If Len(rng.formula) <> 0 Then
         formula = LCase(rng.formula)
         If InStr(formula, "[a") <> 0 Then
            rng = "#Error"
         End If
      End If
   Next
Next
ActiveWorkbook.Sheets(1).Cells(Rows.Count, Columns.Count) = "Checked"

End Sub