The error "Server Error in ‘/’ Application." can display if your site has been recently upgraded.
The production environment has two nodes. During the process of unpublishing customisations, there are scheduled tasks running that may mean the caches for both nodes are not completely cleared. This can result in corruptions when customisations are republished.
Fixing the error
If a single user experiences this error, ask them to try clearing their browser cache.
If all users experience this error, contact your support person so we can clear the server cache.