A folder holding monthly input files can gradually collect an archive, a revised replacement and an exported combined report. A query that reads that whole folder may then ingest material that was never intended as new source data.

Write a short input rule before combining: the required format, reporting periods and location of approved originals. Keep generated outputs outside that input area where practical. An extension check alone cannot distinguish a valid monthly CSV from an old result exported as CSV.

Review the discovered file inventory

The Power Query Folder connector includes files from the selected folder and its subfolders. Select Transform Data to inspect the file table before combining. Microsoft recommends filtering that view using fields such as Extension and Folder Path. The files being combined need a compatible format and structure.

Compare the filtered list with the intended inventory. Check an archive subfolder explicitly and account for any revised file that supersedes an earlier version. Inspect file names as well as paths, since two versions can occupy the same location.

Use a small trial folder containing two approved input files and one deliberately excluded file. Confirm the selected set before processing its contents. This checks discovery separately from the later correctness of the combined rows.

Choose the example that defines extraction

The Combine Files dialog uses an example file, initially the first listed file, and allows choosing another. Power Query creates a sample transformation and a linked function applied to the inputs. Changes to that sample affect the extraction used for the other files. Skip Files with Errors excludes failing files from the output; it does not repair them.

Choose a representative input and verify the intended table or structure within it. A successful sample does not establish that every other file follows its layout. Examine a file from a different reporting period and one with the smallest expected contents.

Reconcile the final result against the approved file list and expected contributions. Investigate any skipped file before describing the report as complete. After a source-schema change, inspect the generated transformation and final output steps again, retaining the input-selection rule so future refreshes do not quietly absorb accumulated outputs.

Sources: Microsoft: folder connector; Microsoft: combining files.