Server Trace Flags
flags are used
to temporarily enable or disable specific
database functions allowing you to change default database behavior
or to observe hidden database behavior and will remain in effect
until they are either manually turned off or SQL Server is
restarted. Keep in mind that most trace flags are undocumented and
Microsoft can and will change the behavior of the flag between SQL
Server versions, service packs or hotfixes.
[Sam Gentile’s Radio

Very useful.