You didn't answer my question as to whether the error, if any, is being returned by a formula so I'll assume it is.
Dim c As Range
On Error Resume Next
With Intersect(Range("F:F"), ActiveSheet.UsedRange)
For Each c In .SpecialCells(xlCellTypeFormulas, xlErrors)
On Error GoTo 0
If c Is Nothing Then Exit Sub
If c.Value = CVErr(xlErrNA) Then