diff --git a/lib/libsys/jail.2 b/lib/libsys/jail.2
--- a/lib/libsys/jail.2
+++ b/lib/libsys/jail.2
@@ -23,7 +23,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd September 9, 2025
+.Dd September 15, 2025
 .Dt JAIL 2
 .Os
 .Sh NAME
@@ -555,6 +555,9 @@
 call
 .Xr chroot 2
 internally, so they can fail for all the same reasons.
+In particular, they return the
+.Bq Er EPERM
+error when the process to join a jail has open directories.
 Please consult the
 .Xr chroot 2
 manual page for details.