Always use checked pre- and post-conditions in a data pipeline.

This simple post-condition would have caught this issue: The sheet after merge operation must have a number of rows equal to the sum of number of rows for all merged sheets.

Assuming this is a merge of a standardized input, then another post-condition might be: The number of columns in output shall equal the number of columns in the input. Might want to check header names, and order as well.

Thinking in terms of universal properties, and putting the checks into production, is better than unit-testing.