javassist.util.proxy.ProxyFactory maintains a cache of proxy classes generated. Apparently it does this based on a static variable. First and foremost, this cache is completely unnecessary given how Hibernate creates and uses these proxy classes. Second there is some conjecture that this might possibly contribute to permgen leaks.
Here is an example of leveraging Javassist without using its ProxyFactory: http://github.com/weld/core/blob/master/impl/src/main/java/org/jboss/weld/bean/proxy/ProxyFactory.java