Here is a list of troubleshooting steps I use for when my Sitecore site is being rude. Hopefully this can help you too!!
โ๏ธ If everything works but getting random 500.19 error
๐น Verify server and framework with coworkers in Web Platform Installer
๐น URL Rewrite can be missing
๐น Could be something in config being referenced, not installed
โ๏ธ Get copy of dllโs, databases
โ๏ธ Unable to add new transform
๐น Install Slow Cheetah โ extensions, not nuget
โ๏ธ IIS error โ verify site setup can connect to other local site that you know works
โ๏ธ Verify connection string and user settings in SQL server
โ๏ธ update-package -reinstall before build
โ๏ธ apppool is set to integrated 4.0.
โ๏ธ check both local and secure port (80, 443)
โ๏ธ SQL user needs to be owner
โ๏ธ Breakpoint for method/action is skipped โ make sure layout isnโt cached
(property in layout)