Important Customer Information about numeric EVENTS
This short note gives important information for any customer wanting to either: 
  A. Add and EVENT="..." line to their initSID.ora startup parameters
 
or
  B. Use an ALTER SESSION / SYSTEM SET EVENTS="..."; command

@or
@  C. Set an event via the DBMS_SYSTEM.SET_EV procedure

Please read the information here before setting any event. If you are not sure about an EVENT then contact your local Support Centre. 
What is an EVENT ?
An EVENT is a special item used by the Oracle server to do one of the following: 
Change behaviour 
Enable collection of trace or debug information 
Enable additional error checking or similar 
When can I set an EVENT ? 
You should only ever set an EVENT if: 
Directed to do so by Oracle Support Services 
Directed to do so by an article or bulletin which applies to the Oracle RDBMS version that you are using AND you understand what the EVENT is going to change. 
It is extremely important that you understand the effects of any event that you set. If you are not sure contact your local Support Centre. 
Other IMPORTANT considerations
1) Make sure the event is valid for the Oracle release you are running
The behaviour of an event can change between Oracle releases.
For example: An event to disable a feature in one release may force a corruption in a different release. 
2) Make sure you know the LEVEL that an event should be set at.
In many cases the LEVEL of an event affects the behaviour of the event. A typical event syntax is:

	EVENT="12345 trace name context forever, level 99"
In this example the EVENT is 12345 and the LEVEL is 99. 
3. If a problem occurs when an event is set then it is worth seeing if the same problem reproduces without the event set. This is because events can enable code specific paths which are not normally used. 

4. Any events set in the initSID.ora file should be re-assessed prior to any upgrade. This is important as the EVENT may have a different meaning in the release you wish to upgrade to. 

Setting an EVENT in INIT.ORA
The syntax to set an event in the initSID.ora file should be specified in the document which tells you to set the event. If not then contact Oracle Support. 
Note that ALL events in the initSID.ora file should be grouped together on successive lines. 


For example: If you have to set event 10046 and event 10053 then
             put the entries on adjacent lines in the initSID.ora thus:

	       EVENT="10046 trace name context forever, level 12"
	       EVENT="10053 trace name context forever, level 1"

It is sensible to always add a comment against any EVENT in the initSID.ora file describing: 
WHAT the event is for 
WHY the event has been added 
WHEN the event was added / by whom / and what the Oracle version was at the time 
WHERE the information about the event came from. 
When removing an event from the init.ora file it is sensible to comment it out with details of WHEN / WHERE / WHY it was removed rather than deleting the lines. .