Challenge 3: Explaining and Fixing an Unqualified Reference — Solution Walkthrough

Why no error occurred:

Cells(i, 4).Value = "Done" is a completely valid VBA statement on its
own — Cells and Range references don't strictly require a worksheet
to be named in front of them, because VBA automatically assumes you
mean the ActiveSheet (whichever tab is currently selected and visible)
whenever no worksheet is specified. The code isn't broken or
malformed in any way; it genuinely does exactly what it says — it
just says less than the colleague actually meant, and VBA has no way
to know that the wrong sheet was active when it filled in that gap.
Since writing "Done" into column 4 of whatever sheet happens to be
active is a perfectly legal operation, VBA has nothing to raise an
error about — the mistake is entirely about WHICH sheet the code
silently chose, not whether the operation itself was valid.

The corrected line:

  Worksheets("Dashboard").Cells(i, 4).Value = "Done"

(substituting whichever worksheet name was actually intended — this
example assumes "Dashboard," matching this chapter's own complete
FlagOverspending example).

Why this fixes it:

Naming the worksheet explicitly removes any ambiguity about which
sheet the code affects — Cells(i, 4) is no longer resolved against
"whatever happens to be active right now," it's resolved against the
one specific worksheet named directly in the statement, regardless of
which tab the user has open when the macro runs. This is exactly the
fully-qualified pattern this chapter's own complete example
(FlagOverspending) uses throughout, precisely to make this class of
mistake structurally impossible rather than just easy to overlook.