The message 

maxproc limit exceeded by uid xxxx, please see tuning(7) and login.conf(5) 

can be misleading when user hits his process limit. In this case notice
about maxproc and tuning is pointless, the message shoud be rather:

user xxxx process limit exceeded, see ulimit -u and login.conf

Short history.

Rev 1.171 (2002) changed handling of user proclimit to "goto fail" -
the same as for maxproc case. Rev 1.203 (2003) added the message with
this comment:
----
Add a ratelimited message of the form
"maxproc limit exceeded by uid %i, please see tuning(7) and login.conf(5)."

Which will be triggered whenever a user hits his/her maxproc limit or
the systemwide maxproc limit is reached.
----

Fix: 

/* Can't copy and clear. */
        if ((flags & (RFFDG|RFCFDG)) == (RFFDG|RFCFDG))
@@ -339,6 +340,7 @@
        sx_xlock(&allproc_lock);
        if ((nprocs >= maxproc - 10 && priv_check_cred(td->td_ucred,
            PRIV_MAXPROC, 0) != 0) || nprocs >= maxproc) {
+               msg = "maxproc limit exceeded by uid %i, please see tuning(7) and login.conf(5).\n";
                error = EAGAIN;
                goto fail;
        }
@@ -359,6 +361,7 @@
                PROC_UNLOCK(p1);
        }
        if (!ok) {
+               msg = "user %i process limit exceeded, please see ulimit -u and login.conf(5).\n";
                error = EAGAIN;
                goto fail;
        }
@@ -803,8 +806,7 @@
 fail:
        sx_sunlock(&proctree_lock);
        if (ppsratecheck(&lastfail, &curfail, 1))
-               printf("maxproc limit exceeded by uid %i, please see tuning(7) and login.conf(5).\n",
-                   td->td_ucred->cr_ruid);
+               printf(msg, td->td_ucred->cr_ruid);
        sx_xunlock(&allproc_lock);
 #ifdef MAC
        mac_proc_destroy(newproc);--SmZwCCc4Iz80HB9RjZ98hzP3z8maNe5dCMZq2N6EMM21q3Ml
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--- kern_fork.c.old     2011-04-12 15:04:57.000000000 +0200
+++ kern_fork.c 2011-04-12 15:08:48.000000000 +0200
@@ -216,6 +216,7 @@
        struct vmspace *vm2;
        vm_ooffset_t mem_charged;
        int error;
+       const char *msg;