In the project I'm working on now, we use XML documentation in our code. This morning we tried to compile a release mode version of the project, because we needed to build a test setup version.
The compiler suddenly complained about missing XML comments. When we checked the source we found this:
// ///
// /// Creates an instance of the Crypt class
// ///
A programmer, who will remain anonymous, commented the XML comments! He had problems compiling the application (like we now have) and tried to solve it by commenting the XML comments.