As my train wasn't due to leave for another hour, I had plenty of time to spare. After buying some newspapers to read on the journey, I made my way to the luggage office to collect the heavy suitcase I had left there three days before. There were only a few people waiting, and I took out my wallet to find the receipt for my case. The receipt didn't seem to be where I had left it. I emptied the contents of the wallet, and railway-tickets, money, scraps of paper, and photographs fell out of it; but no matter how hard I searched, the receipt was nowhere to be found.
When my turn came, I explained the situation sorrowfully to the assistant. The man looked at me suspiciously as if to say that he had heard this type of story many times and asked me to describe the case. I told him that it was an old, brown-looking object, no different from the many cases I could see on the shelves. The assistant then gave me a form and told me to make a list of the chief contents of the case. If they were correct, he said, I could take the case away. I tried to remember all the articles I had hurriedly packed and wrote them down as they came to me.
After I had done this, I went to look among the shelves. There were hundreds of cases there and for one dreadful moment, it occurred to me that if someone had picked the receipt up, he could have easily claimed the case already. This hadn't happened fortunately, for after a time I found the case lying on its side high up in a corner. After examining the articles inside, the assistant was soon satisfied that it was mine and told me I could take the case away. Again I took out my wallet: this time to pay. I pulled out ten-shilling note and the "lost" receipt slipped out with it. I couldn't help blushing and looked up at the assistant. He was nodding his head knowingly, as if to say that he had often seen this happen before too!
What dreadful idea occurred to the writer? ____.（ ）
参考文章第三段第二句。
