The ProxySG appliance event log shows the following message:
Unexpected disposition by "HTTP RW XXXXXXXXX" for "http://xxx.xxx.xxx/" 0 48040:1 cea.cpp:3943
This message might indicate that the retrieval/refresh worker deletes an object of the URL and then marks it as non-cacheable. It is an internal error that has no negative impact on the SGOS performance and thus can be safely ignored.