Handling Errors and Warnings in Formulas
When a formula in a table cell is incomplete, contains invalid cell references, or is
otherwise incorrect, or when an import operation creates an error condition in a
cell, Numbers displays an icon in the cell. A blue triangle in the upper left of a cell
indicates one or more warnings. A red triangle in the middle of a cell means that a
formula error occurred.
To view error and warning messages:
Click the icon.
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A message window summarizes each error and warning condition associated with
the cell.
To have Numbers issue a warning when a cell referenced in a formula is empty, choose
Numbers > Preferences and in the General pane select “Show warnings when formulas
reference empty cells.”