sneaky dual-boot

Done that mistakenly, on several occasions. It is *incredibly* confusing! The machine hardly ever gives any sort of on-screen indication of how it's booting, so the symptom is often that you install one OS, reboot, and the OS the machine used to run leaps into action as if it was not installed over, with the new one invisible (because it's using a completely different partitioning scheme) and with the old one often misbehaving (because parts of it have just been overwritten more or less completely at random).

This fubar is becoming rarer now because few machines with existing DOS partition tables are getting GPT-reformatted, and the installers that do that are using wipefs or equivalent and thus are *actually* wiping out the old one properly. But oh good grief was it confusing for a few years.