Customer: "The download URL generated by our website is not working."
Me: "TBH it has never worked."
Customer: "Yes it has!"
Me: "OK, see the plus signs in this+is+my+file.pdf that is not a legal character in a URL it should be %20"
Me: "I've turned on directory listing on on your website. Can you see all the documents now?"
Customer: "Oh yeah."
Me: "So you would have got myfile.pdf but not my file.pdf you would have got anotherfile.pdf but not yet another file.pdf"
Customer: "Oh, that explains why sometimes it worked and sometimes it didn't."