The problem seems to arise from other parts of your code. Could you post the
whole sub, please so we can see what it is doing?

I was working with this issue and found tht if i Change

Target.FormulaHidden = True TO

Range(Target.MergeArea.Address).FormulaHidden = True ,

works fine  if i use backspace  to delete  the contents. but this vl
show error  msg if i use Delete key  to delete the cell  contents.

Here is the code,

Private Sub Changed(ByVal Target As Range, Optional what As String)

Dim rowOffset As Integer
If IsEmpty(Cells(Target.Row, Target.Column)) Then

If (what <> "") Then
Select Case what
Case "Add"
Target.Formula = "=A+B"
rowOffset = 22
Case "Sub"
Target.Formula = "=C-D"
rowOffset = 23
End Select
Else
If ((Target.Row - 1) Mod (Max + Max + 1) < AddAcc + 2) Then
Target.Formula = "=A+B"
Else
Target.Formula = "=C-D"
End If
rowOffset = 0
End If

Target.Font.Italic = True
Target.Font.Bold = False

Target.FormulaHidden = True 'this line shows  the error.

Target.Offset(-1, 0).Select

Else

'Selecting next cell down
Target.Offset(1, 0).Select

End If

End Sub

I cannot identify the problem. I would suggest you set  a
breakpoint on the instruction and run  the macro. When it stops, go into debug
and look at what the target is, what its contents  are, etc.

I am assuming protection is turned off for the worksheet at this point. If it
is turned on, you may need to turn it off before the instruction and on again
afterwards.

I added breakpoint and checked it out. When i use DEL key  to delete
the contents  of the cell, the value of Target.Address will be
$C$168:$E$168.
Same time  if i use backspace  to delete  the contents, the value of
Target.Address is $C$168. and i think this is where the problem is.

I am still not able to find the solution and working with it.

Untested, but try ...

Target.Address.Range("A1")

This should give you $C$168 no matter what came in.