You may find a lot of entries with "gc freelist" in the oracle logs. This is known bug 18628388.
This bug is only relevant when using Real Application Clusters (RAC).
GCS cache lock is not closed fast enough and "gc freelist" wait appears on heavy load system.
If you see "gc freelist" wait occupies major part of database wait events, you might see this problem.
Patch can be obtained thru MyOracleSupport.