In this case the server app had multiple DLLs imported from other projects and they had been compiled against a specific version of the Oracle .net driver.

My docs clearly stated that you should not update the driver unless you were willing to rebuild all the other projects as well. I could only deploy one copy of the driver file, so it had to be the version all the different parts of the application were expecting. It was an old app, whoever started development on it didn't know about repositories and DB singletons - there were DB connections all over the place.

The annoying thing is that the .net error message they were getting actually stated what the problem was in fairly clear terms, yet they still called me.