For example, you should NEVER install both Bitcoin Core and Bitcoin ABC (Bitcoin Cash) in the same computer. Those are coded with the same path to the data directory.
Minor correction: they're both coded with the same DEFAULT install and data directory paths. Both of which are easily customisable.
You can safely run Bitcoin Core and Bitcoin ABC on the same PC. I currently do this and as long as you set them up properly, and don't just blindly click "next" during install and setup, they'll happily run side by side without interfering with each other