Comment by tgv

By coincidence, I found another reason just two days ago. A web app lists uploaded files’ names, and (in a rarely used context) lets the user search for them. One user has copied a file name from the web page, and pasted it into the search box, but got no results. Turned out that the file name contained two consecutive spaces, which the browser turns into a single space, hence no match. Every layer between the user and file system can do something unexpected.