Measuring the pool

Text: an original contemporary personal account-style practice extract.

When I helped set up the school swimming event, I checked the lane numbers against the printed race list. Two labels had been swapped while the ropes were being cleaned. I moved them back before swimmers arrived. The pool attendant tested the starting signal, while another helper arranged chairs. Later, a teacher asked why our preparations had taken so long. I pointed to the corrected numbers: a few minutes checking an empty pool seemed preferable to sorting out a disputed result with thirty wet competitors waiting.

From the extract, identify one error the writer corrected before swimmers arrived.