From max.ross on March 18, 2008 00:19:58

All enhanced classes should implement a marker interface so that client
code has a reliable way to determine if an object is an instance of a
Guice-enhanced class.  Ideally the interface(s) would be configurable by
the client, but if Guice just has its own interface that its proxies always
implement that would be cool too.

Original issue: http://code.google.com/p/google-guice/issues/detail?id=187