You must have grouped the worksheets (e.g. by having the left-most one
selected and then shift-clicking the tab of the right-most one).
[Group] will show in the title bar when it is in this state.

You could put this code in the ThisWorkbook module to be warned as it
happens:

Private Sub Workbook_SheetChange(ByVal Sh As Object, ByVal Target As
Range)
If ActiveWindow.SelectedSheets.Count 1 And Sh.Name =
ActiveSheet.Name Then
MsgBox "You just made a change in group mode" & vbNewline & _
"it will affect other sheets; edit/undo if it was not inteded"
End If
End Sub

I hope you had a backup.

Bill Manville
MVP - Microsoft Excel, Oxford, England
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