Q:Since you've released your application to production, you've had lots of 
deadlock problems in your database on a random basis. You want to trap 
deadlocks and output detailed information to the SQL Server log. What trace 
flag could you enable in the startup parameters of SQL Server to accomplish 
this task?

Answer: 1204

Explanation:
If a deadlock situation continues, it is often useful to use trace 
flag 1204 to gather more information. Trace flag 1204 prints out 
the deadlock chains and victim, as shown in this sample output: 


*** Deadlock Detected ***
 ==> Process 7 chosen as deadlock victim
 == Deadlock Detected at: 1998-09-10 16:39:29.17
 == Session participant information:
 SPID: 7 ECID: 0 Statement Type: UPDATE
 Input Buf: update t1 set c1 = c1  where  c1 = 2

 SPID: 8 ECID: 0 Statement Type: UPDATE
 Input Buf: update t1 set c1 = c1  where  c1 = 1


 == Deadlock Lock participant information:
 == Lock: KEY: 2:117575457:1 (010001000000)
 Database: tempdb
 Table: t1
 Index: i1
  - Held by: SPID 7 ECID 0 Mode "S"
  - Requested by: SPID 8 ECID 0 Mode "X"
 == Lock: KEY: 2:117575457:1 (020002000000)
 Database: tempdb
 Table: t1
 Index: i1
  - Held by: SPID 8 ECID 0 Mode "S"
  - Requested by: SPID 7 ECID 0 Mode "X"

This deadlock information can be interpreted as follows: 

The first section displays the deadlock victim and time of deadlock, along with 
the sessions involved in the deadlock. For each session, the current SPID, 
statement type, and a portion of the input buffer are displayed. 

The second section displays details about the locks involved in the deadlock. 
From the output above, note that the deadlock involves key locks on table t1, 
index i1. The deadlock output shows which processes own the locks involved in 
the deadlock and which sessions are waiting for the locks to be granted as well 
as the associated lock modes. 

The process that has generated the least amount of log volume will, by default, 
be chosen as the deadlock victim and be rolled back automatically. To influence 
which session is rolled back, set the DEADLOCK_PRIORITY for a session. 

Reference : 1205 (error) in Books Online