diff --git a/doc/misc/gnus.texi b/doc/misc/gnus.texi
index b3847647406..4559ae8e31e 100644
--- a/doc/misc/gnus.texi
+++ b/doc/misc/gnus.texi
@@ -22727,6 +22727,13 @@ output lines in the various buffers.  There's quite a lot of them.
 Fortunately, they all use the same syntax, so there's not that much to
 be annoyed by.
 
+Gnus does not use the font locking machinery used by most modes in
+Emacs, so switching @code{font-lock-mode} on in the Gnus
+group/summary/article buffers usually doesn't do anything
+useful---instead it'll just mess up the faces that Gnus has already
+put in the buffer.  (This is also the case for other minor modes that
+use the font locking machinery, like @code{whitespace-mode}.)
+
 Here's an example format spec (from the group buffer): @samp{%M%S%5y:
 %(%g%)\n}.  We see that it is indeed extremely ugly, and that there are
 lots of percentages everywhere.