What determines whether a coded exception in a trigger or SP is written to the log file?

Most exceptions are returned to the client, and do not appear in the log, but occasionally an exception is written in full to the log file. What causes this?

Here is an example, from a live appplication. The PSQL which creates the exception is

IF (COALESCE(ALLOC_TOTAL,0) > BATCH_QTY) THEN
EXCEPTION FLEX_EXCEPTION 'Sales allocation of '||COALESCE(VENDOR_DESCRIPTION,'')||' exceeds the purchase';

and this is the output in the log file.

NFPSERVER (Server) Fri Mar 13 08:07:06 2009
Database: NFP
exception 3
FLEX_EXCEPTION
Sales allocation of Narcissi Tete a Tete exceeds the purchase
At procedure 'PURCHASE_LINE_TRIG4' line: 48, col: 44
At trigger 'PURCHASE_LINE_AI' line: 9, col: 5


Roger Vellacott
Passfield Data Systems Ltd