Challenge 3: Diagnosing and Fixing a Retriggering Worksheet_Change — Solution Walkthrough

The likely original handler:

Private Sub Worksheet_Change(ByVal Target As Range)
    If Target.Column = 3 Then
        If Target.Value > 1000 Then
            Target.Offset(0, 1).Value = "Warning: High Value"
        End If
    End If
End Sub

Diagnosis:

Entering a value over 1000 into column C fires Worksheet_Change once,
with Target pointing at that column-C cell. The condition passes, and
the code writes a warning into column D (Target.Offset(0, 1)). That
write is ITSELF a change to the worksheet, which fires
Worksheet_Change a second time, with Target now pointing at the
column-D cell that was just written to. On that second firing,
Target.Column = 3 is FALSE (column D is column 4), so the inner logic
never runs again and the chain stops there — this is technically NOT
a true infinite loop, since it doesn't keep re-triggering forever.

However, it IS a genuinely wasteful, redundant extra event firing
every single time a qualifying value is entered — VBA re-evaluates
the entire Worksheet_Change procedure a second time for no useful
purpose, and depending on what else is in the workbook (other event
handlers, volatile formulas recalculating, screen redraws), that
redundant firing is very plausibly what produces the brief
unresponsiveness the colleague is noticing, especially on a workbook
with other automatic processes chained to worksheet changes.

The more serious risk this exact pattern sits right next to: if the
Offset were ever changed to point back at column C itself (or any
future edit accidentally did), the second firing's Target.Column = 3
check WOULD pass, writing again, re-triggering a third time, and so
on — a genuine infinite loop, one small typo away from this exact
code.

The fix — wrap the write in EnableEvents:

Private Sub Worksheet_Change(ByVal Target As Range)
    If Target.Column = 3 Then
        If Target.Value > 1000 Then
            Application.EnableEvents = False
            Target.Offset(0, 1).Value = "Warning: High Value"
            Application.EnableEvents = True
        End If
    End If
End Sub

Setting EnableEvents to False immediately before the write means that
write does not fire Worksheet_Change at all — the redundant second
call never happens, and the more serious future risk (a typo turning
this into genuine infinite recursion) is eliminated at the same time,
since NO write made inside this block can ever re-trigger the handler
regardless of which column it lands in. Setting it back to True
immediately afterward ensures the very next genuine user edit still
fires the handler normally.