After our server minions did some MS SQL database restores from some old backups to find some data, some .NET websites we had on the server started showing an error.
But I couldn't view the error remotely. Oddly this worked locally, and I couldn't get the error for some reason, even in a web browser on the server via remote desktop.
So, I needed to show the error temporaily on that live server, so in the inetpub/websitename folder, locate the web.config file and do this: https://technet.microsoft.com/en-us/library/bb684665.aspx
Namely: Change the customErrors mode attribute from "RemoteOnly" to "Off", and then save the file.
Refresh the page and get the error, then turn this setting back off.