Google Sheets formulas silently break in Excel, hiding errors behind IFERROR fallbacks

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When a Google Sheet is exported to .xlsx format, Google-specific functions like ARRAYFORMULA, QUERY, and FILTER do not exist in Excel and fail to evaluate. Rather than displaying an error, Excel wraps these unrecognised functions as DUMMYFUNCTION placeholders, which are then caught by any outer IFERROR logic in the original sheet. This causes cells to silently return fallback text — such as a dash or 'no matches' — making the file appear healthy when it is not. A developer discovered 203 such affected cells in a file being distributed via openpyxl, prompting them to write a Python script to detect Google-only function strings in formula cells. The recommended fix involves replacing dynamic-array functions with universally compatible alternatives like SUMPRODUCT and INDEX/MATCH, which work across Excel 2016, 2019, and 365.

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