Ask HN: Do you have a server troubleshooting guide?

I'm learning how to manage linux servers and the amount of topics to keep track of is pretty daunting (Apache / Nginx, dbs, mx, DNS, networking, CDNs, server load, etc). I'm putting together my own troubleshooting guide and I'd love to know what personal / public guides / checklists HN uses when encountering issues while doing SysAdmin.