From:
http://docs.redhat.com/docs/en-US/JBoss_Enterprise_Data_Services/5/html/Data_Services_Caching_Guide/results.html#id2656418

Section 2.2.1, example 2.2
/*+ cache(pref_mem ttl:60000 */ select col from t

is missing a right paren and should read:
/*+ cache(pref_mem ttl:60000) */ select col from t

Copying this example from the docs will cause the cache hint to be invalidated.