Windows Server, IIS, Linux, and the systems that host everything else.
What an IIS application pool actually is, why it stops or crashes, and how to tell the difference between an app pool problem and an application code problem.
How Linux file permissions actually work — the owner/group/other model, reading and writing permission strings, and the chmod/chown commands to change them safely.
A practical, non-overwhelming approach to Windows Event Viewer — which logs actually matter, how to filter out the noise, and how to find the one entry that explains a crash.