Tomcat - Unable to locate persistence units error

I recently was working on updating the libraries for a Java web application and was unable to connect to the database. Stepping through the program in Tomcat I saw that it was giving me the "Unable to locate persistence units" error. This puzzled me for a long time, so I decided to take a closer look as I stepped through the application. The actual error message in the exception was "unrecognized persistence.xml version 2.2"! So I reverted back to version 2.1!