Other than a unprivileged user being able to bring a server to it's knees with a simple dd (or cp) command?
Just find out the user number assigned to your user...
dd if=/dev/zero of=/run/user/<usernumber>/test.txt bs=512 count=999999999
It will fill the /run tmpfs up and then system processes/services can't use it.
This can be run by a normal user that has ssh'd into the server as well as a local user. No root privilege necessary.