I have recently noticed "Agent Internal: Reflection Errors" in AppDynamics.
For my, specific, application, I'm seeing hundreds of errors per minute on average, but the error seems to peak at 8,000 or so errors every few minutes and average out to a few hundred every minute.
When I look through the history, I noticed that they started on February 10th, 2016.
"Reflection Errors" seem to be Java specific, so I checked all Java nodes in the applications I have access to and all of them have similar, large number of Agent Internal: Reflection Errors. There are no more details than the number per minute and the graphs. There is no way to look inside the errors themselves.
Screenshots attached.