Have you ever come across an issue, like below.
Assume that you are working in a team, and one of your team member, commit all his work (both code files, and Sitcore items) to version control, You have taken latest, and sync all Sitecore items to your local, publish, and tries to load the page.
Boom, you are welcomed with System.NullReferenceException: Object reference not set to an instance of an object.
I face this issue [in]frequently. This always ends up with Unknown Rendering issue. Apparently your team member may forgot to check-in the rendering item, but check-in the content item, which references the rendering.
Whenever you come across this issue, you can always start from checking the item presentation details for any Unknown rendering,
Happy Troubleshooting :)