Github user papegaaij commented on the issue:
https://github.com/apache/wicket/pull/233
We don't care about concurrent calls to valueUnbound, that's fine. All code
in that method is thread safe. What we don't want is multiple calls to
storeTouchedPages and valueUnbound (session expiry) to interfere with each
other. Synchronizing on the entry will not help as session expiry is probably
done from a different thread.---
