We have found that we have been dropping databases without dropping the backup files in the Fast Recovery Area (FRA).
We have been using the DROP DATABASE command from SQL*Plus, and have found that we have accumulated several terabytes of unowned backups in the FRA.
A much better way to drop a database is to use the RMAN version of the command as shown in the following example (from the RMAN documentation):
CONNECT TARGET / STARTUP FORCE DBA MOUNT DROP DATABASE INCLUDING BACKUPS NOPROMPT;
The documentation also reminds one to be wary of the NOCATALOG mode as backups known to the RMAN catalague, but not known to the control file, are kept. In other words, in order to delete all known backups, run these commands in CATALOG mode.
Update 2016/01/01 Corrected comment about NOCATALOG and CATALOG modes.