A friend of mine, that I’ll keep anonymous, asked me to help her with an ASP page that didn’t work. The changes she made wasn’t visible when she ran the script on the server. I got most of it running on my local machine and thought there should be no problem running it on the production server. A couple of hours later the ASP file contained nothing but plain HTML and it still wasn’t working. By this time she was convinced the web server was serving an old file to visitors. (No it wasn’t a caching issue.) I didn’t hear from her in about two hour until she told me the issue was solved. Clearly a bit ashamed she told me she had an intern working for her. They had been working on the same file and none of them had a development server, so they were both uploading their versions of the same file to the server.
Not considering the lack of coordination and development environments, they definitively would have been better off using version control.