Family tech support of the day - a system completely crashed after the C: drive was filled up by 50 GiB of WeChat history, it just hung here with a black screen after logging in. After the data was relocated, It was able to enter the desktop immediately. But the full partition already caused significant damages to system files, like corrupting the HKLM\COMPONENTS and HKLM\DRIVERS sections of the registry.

A full system partition is a truly cross-platform way to run a system to the ground, I've seen it on Unix servers and a Xen Dom0, now it's Windows.