Good point but just in case if the user has not been using a customised UI, maybe everything should look non-default and out of place. If I remember correctly, the system defaults to a low resolution when something goes wrong and this may help.

The point is, the user should be made aware that this is not business as usual and proceed with extreme caution. A popup with cryptic title and long paragraph won't do it.

On some Linux distros logging in with the root account gives you a lot of red in the ui. Same as the terminal colours when you sudo su.

Since Windows is giving you a temporary profile that doesn't have any of your settings, why not put a warning on the wallpaper, make the directories read only, the ui red/yellow, or add a logoff script that checks pictures, videos and docs directories etc? Back in my windows days I did a ton of user profile configuration for roaming profiles, default profiles etc. This stuff isn't hard. You'd expect Microsoft to do a better job at it when there's a risk of data loss