I have a project which was using Elmah.Mvc.dll, but I removed it from my project references.
A deploy to DEV (IIS on a local VM) showed that it was successfully removed
A deploy to PROD (Azure website) gave me errors, then I noticed that the file (Elmah.Mvc.dll) was still in the bin directory in my Azure website although all other updates were correct.
After removing the file through the VS Azure browser, everything started working fine.