A cooler journey

Original non-fiction-style practice text.

Our fish wholesaler tested three insulated containers before choosing the smallest. It kept the fish cold for the entire journey to the market and cost less to transport than either of the larger boxes. Packing the van became quicker too: the identical containers fitted into every available space. For several weeks, the arrangement worked exactly as planned.

Then a failed starter motor left the van outside the depot until nearly noon. The fish had been packed at dawn, and the normal journey took only forty minutes. The container test had never included several extra hours in a stationary vehicle. By the time a replacement van arrived, the supervisor had rejected the load after checking its temperature.

She ordered another trial before changing the containers. This time, staff measured how long each box remained cold beyond the usual delivery time. They also kept a spare vehicle available on the busiest mornings. The owner complained that these precautions would cost money even on days when nothing went wrong. The supervisor agreed, but pointed out that an empty space or an unused vehicle could still be doing useful work. The former plan had been efficient only while every part of the morning ran on schedule. Its weakness had stayed invisible because nobody had yet asked it to cope with a delay.

Use your own words to explain what the phrase “its weakness had stayed invisible” means in the final paragraph.