I built a website for a client which included a page where visitors can download a zipped folder with a few files. A few months later I got an email.

Client: There’s a problem. The site doesn’t just download the file, it downloads all these other files that visitors should NOT have access to. We need to fix this ASAP.  

I checked and it was working fine.

Me: Everything looks fine here. What files are you seeing it is downloading?

Client: Well I see all of these other things here like files called Desktop, and favorites, and documents and downloads….

She was seeing all of the files on her computer.

Source: Clients from Hell