I have the following SQL statements, dynamically created. The value
of 12 is for illustration purposes only:

UPDATE IBAT SET IB_QTY=IB_QTY-12 WHERE CODE='A'
UPDATE IBAT SET IB_QTY=IB_QTY-+12 WHERE CODE='A'

Both the above work fine. However:

UPDATE IBAT SET IB_QTY=IB_QTY--12 WHERE CODE='A'

does nothing whatsoever, no update, no exception, just nothing.

This works:
UPDATE IBAT SET IB_QTY=IB_QTY- -12 WHERE CODE='A'

FB 1.5.3 and FB 2.0 show the same behaviour.
Tested using D5 and IBX, but also using IB_SQL.

Peter
Technisoft