Inexperienced person seems to have done everything fine so far. This is his very first time that he is the only one in charge for LAMP server.

He has installed OS, network, Apache, PHP, MySQL, Proftpd, MTA & MDA software, configured VirtualHosts properly (facts because he calls himself admin), done user management and various configuration settings with respect to security recommendations and... everything is fine for now... For now.

If you were directing horror movie for server admin above mentioned what would you make up for boogieman that showed up and started to pursue him?

Omitting hardware disaster cases for which one cannot do anything 'from remote',

what is the most common causes of server or part-of-server or server-related significant failure when managed by inexperienced admin?

I have in mind something that is newbie admins very often missing which is leading to later intervention of someone with experience?

May that be some uncontrolled CPU-eating leftover process, memory-related glitch, widely-used feature that messes up something unexpected on anything like that?

Newbie admin for now only monitors disk-space and RAM usage, and number of running processes. He would appreciate any tips regarding what's probably going to happen to his server over time.