I'm trying to reinstall Ubuntu 15.04 but I am stuck on bug 946663. What do I need to do to safely cancel the installation process in the middle of it? If I just hold the power button down, will that screw with any of my data?
If I just hold the power button down, will that screw with any of my data?
No, unless the data was screwed to begin with (for instance due to you telling the system to format the data) all a reboot/shutdown will do is stop the installation. You then can restart the installation. Again though: formatting/repartitioning will be a permanent change to the system during the installation.
WARNING: The above is only true for this particular case. Holding down the power button on an already installed system is dangerous and will render Ubuntu inoperable.