I came across a bug in the sourceforge web server, where it refuses connections from my application. I have a small text file on the project web page that contains version information. The application simply fetches that text file to see whether a new version is available. At some point, all users of my application started getting errors that they couldn't retrieve update information.
The problem seems universal across all of sourceforge, and is not limited to the project web space. For example, this will fail:
wget --user-agent Mozilla/5.0 https://sourceforge.net
I don't know what's so special about Mozilla/5.0 as user-agent, but it triggers the bug. It's probably the most compatible user-agent string out there for simple HTTP, and some network stacks use it as default. The network stack in this case is Qt's network module, which uses "Mozilla/5.0".