Re:existing state has been invalidated.. best way to trouble shoot?
The cause for this is the SGA (memory that is allocated by an Oracle Instance and is shared among Oracle processes). Any object in SGA referencing an object that is compiled become invalid during this compilation.
So in this case the process accessing the compiled object needs to be restarted. For example, in case of Oracle Workflows, this error can be rectified by restarting the Workflow Mailer ( or a Retry also helps). One more way is to bounce the database itself which ofcourse will be a last resort.