Say you've got a purely static website. It's popular so people are often visiting your index.html. What happens when you use your ftp client to overwrite the file? Will people ever see any odd results? Will all requests simply hang until the writing is done? Or do webservers do anything clever to serve the old file while the new file is being written?
What if it's a script that overwrites index.html? Is that the same as using ftp? Does it depend on the server?