When shutting down ReactOS, it does not actually free the cluster chain occupied by the pagefile.sys file of the session. This results in rapid free space depletion, though apparently enough space is left free for the operating system to function when the free space is critically low.
Observed with commit ca744679045e7f3615db588a8223ace2058e3f10
1. Install ReactOS to a VM with 4GB RAM, 10GB disk.
2. Reboot a couple times
3. Check free space on C:
4. Will be around ~250MB free.
Also run fsck.fat/chkdsk externally, it verifies that the cluster chains weren't freed and fixes the issue in the file system.