Here is a list of troubleshooting steps I use for when my Sitecore site is being rude. Hopefully this can help you too!!
✔️ If everything works but getting random 500.19 error
🔹 Verify server and framework with coworkers in Web Platform Installer
🔹 URL Rewrite can be missing
🔹 Could be something in config being referenced, not installed
✔️ Get copy of dll’s, databases
✔️ Unable to add new transform
🔹 Install Slow Cheetah – extensions, not nuget
✔️ IIS error – verify site setup can connect to other local site that you know works
✔️ Verify connection string and user settings in SQL server
✔️ update-package -reinstall before build
✔️ apppool is set to integrated 4.0.
✔️ check both local and secure port (80, 443)
✔️ SQL user needs to be owner
✔️ Breakpoint for method/action is skipped – make sure layout isn’t cached
(property in layout)