diff --git a/lib/geom/label/glabel.8 b/lib/geom/label/glabel.8
--- a/lib/geom/label/glabel.8
+++ b/lib/geom/label/glabel.8
@@ -22,7 +22,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd March 12, 2017
+.Dd September 27, 2023
 .Dt GLABEL 8
 .Os
 .Sh NAME
@@ -141,6 +141,26 @@
 are created in the
 .Pa /dev/label/
 directory.
+Note that generic, automatic labels occupy some space on the device
+and thus should not be added to a device already containing a file system.
+In particular,
+.Nm
+reserves the last sector of the device to store the label information.
+If the device already contains a file system,
+.Nm
+will overwrite the last sector, possibly damaging the file system, and the
+file system may later overwrite the label sector.
+Instead, create a label before initializing the file system, and initialize
+that file system on the device created by
+.Nm
+under the
+.Pa /dev/label/
+directory.
+Then the file system will correctly account for the space occupied by the
+generic label,
+since the
+.Nm
+device will be one sector smaller than the device from which it was created.
 .Pp
 Note that for all label types, nested GEOM classes will cause additional
 device nodes to be created, with context-specific data appended to their