Major Description
The object passed for checking could be resolved by calling its #writeReplace() method at https://github.com/apache/wicket/blob/379f9e54359be17619be28b0b6303dc9ce3a8e81/wicket-core/src/main/java/org/apache/wicket/core/util/objects/checker/CheckingObjectOutputStream.java#L374.
Later the the original instance should be put in the "checked" map because otherwise it might fail with StackoverflowError.
I our case we have a JPA entity User that has a member field "modifiedBy". When this user is an admin it is possible that it modifies itself. So the original checked instance is a Javassist proxy, after #writeReplace() it is a plain object. Until now the plain object is "checked" and then when its fields are processed the "modifiedBy" is again the same Javassist proxy and the processing continues in loops.