Question on ora-63999

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We encountered a production database crash with ORA-63999 error;

When reproduced the problem in the testing env, the observation we have on an user session which running a particular process, monitor v$tempseg_usage, there is LOB_DATA type keep growing to say 3GB, the tempfile, with current size say only 10M (sparse file, newly created for the test) sitting on a filesystem with only, say 2GB; The tempfile didnot grow, until we did "alter system flush buffer_cahe"; the tempfile start to grow, until it consumed all the remaining 2GB of the filesystem and then DBWR crashed which in turn bring down the database instance.