I came accross recently a very weird issue on Oracle WebCenter 11g PS4 (126.96.36.199).
I had a page, with four portlets. One of them had a portlet parameter. When I tried to provide a value for that portlet parameter at runtime (using the WebCenter Administrator), i got a ConcurrentModificationException exception. Here is a snippet from the log file:
After loggin an SR, this is a known issue and Oracle has a fix for it (patch 10427778).