Kubuntu uses Launchpad to keep track of bugs and their fixes. To file a bug you first need to create an account. Once you have an account you can use the tools we've provided you on your Kubuntu system to file bug reports. For the desktop environment and most applications, the KDE Bug Tracker is also used. Reporting bugs there will get them closer to the actual developer of the application, but be wary of bugs that may be specific to Kubuntu. If you aren't sure, please file bugs in Launchpad first.