This article is from the Macintosh hardware FAQ, by Elliotte Rusty Harold firstname.lastname@example.org with numerous contributions by others.
All Sony Trinitron monitors have a stabilizing wire in position to
cast a shadow about three quarters of the way down the screen. Larger
monitors also have a wire about a third of the way down from the top
of the display. There is no way to fix this short of redesigning the
Trinitron tube. This line is more obvious on some monitors than
others so if it seems particularly bad you can try to convince the
dealer you bought it from to exchange the monitor. Other than that
there's nothing that can be done about it. Other than that there's
nothing that can be done about it.