The range that stopped one row short

A range is a fixed pair of corners. Write =SUM(B2:B6) and it means those five cells for ever, regardless of what gets added beneath them.

So the moment somebody types a sixth figure into B7, the total quietly stops being the total. There is no error, no highlight, nothing out of place, just a number that is now too small, sitting under a column that looks complete.

This is the single most common way a working spreadsheet becomes wrong, and the only defence is the habit of checking the range when the data grows.