diff --git a/opal/util/cmd_line.c b/opal/util/cmd_line.c
index 1adc891b27..4be4fe95c0 100644
--- a/opal/util/cmd_line.c
+++ b/opal/util/cmd_line.c
@@ -267,7 +267,8 @@ int opal_cmd_line_parse(opal_cmd_line_t *cmd, bool ignore_unknown,
     int i, j, orig, ret;
     cmd_line_option_t *option;
     cmd_line_param_t *param;
-    bool is_unknown;
+    bool is_unknown_option;
+    bool is_unknown_token;
     bool is_option;
     bool has_unknowns;
     char **shortsv;
@@ -310,7 +311,8 @@ int opal_cmd_line_parse(opal_cmd_line_t *cmd, bool ignore_unknown,
     option = NULL;
     has_unknowns = false;
     for (i = 1; i < cmd->lcl_argc; ) {
-        is_unknown = false;
+        is_unknown_option = false;
+        is_unknown_token = false;
         is_option = false;
 
         /* Are we done?  i.e., did we find the special "--" token?  If
@@ -328,11 +330,12 @@ int opal_cmd_line_parse(opal_cmd_line_t *cmd, bool ignore_unknown,
             break;
         }
 
-        /* If it's not an option, then we've found an unrecognized
-           token. */
+        /* If it's not an option, then this is an error.  Note that
+           this is different than an unrecognized token; an
+           unrecognized option is *always* an error. */
 
         else if ('-' != cmd->lcl_argv[i][0]) {
-            is_unknown = true;
+            is_unknown_token = true;
         }
 
         /* Nope, this is supposedly an option.  Is it a long name? */
@@ -371,11 +374,11 @@ int opal_cmd_line_parse(opal_cmd_line_t *cmd, bool ignore_unknown,
                         opal_argv_insert(&cmd->lcl_argv, i, shortsv);
                         cmd->lcl_argc = opal_argv_count(cmd->lcl_argv);
                     } else {
-                        is_unknown = true;
+                        is_unknown_option = true;
                     }
                     opal_argv_free(shortsv);
                 } else {
-                    is_unknown = true;
+                    is_unknown_option = true;
                 }
             }
 
@@ -388,9 +391,9 @@ int opal_cmd_line_parse(opal_cmd_line_t *cmd, bool ignore_unknown,
 
         if (is_option) {
             if (NULL == option) {
-                is_unknown = true;
+                is_unknown_option = true;
             } else {
-                is_unknown = false;
+                is_unknown_option = false;
                 orig = i;
                 ++i;
 
@@ -496,9 +499,9 @@ int opal_cmd_line_parse(opal_cmd_line_t *cmd, bool ignore_unknown,
            handle it.  Copy everything (including the current token)
            into the tail.  If we're not ignoring unknowns, then print
            an error and return. */
-        if (is_unknown) {
+        if (is_unknown_option || is_unknown_token) {
             has_unknowns = true;
-            if (!ignore_unknown) {
+            if (!ignore_unknown || is_unknown_option) {
                 fprintf(stderr, "%s: Error: unknown option \"%s\"\n", 
                         cmd->lcl_argv[0], cmd->lcl_argv[i]);
                 printed_error = true;
diff --git a/opal/util/cmd_line.h b/opal/util/cmd_line.h
index def93187a0..104f91e366 100644
--- a/opal/util/cmd_line.h
+++ b/opal/util/cmd_line.h
@@ -365,11 +365,19 @@ BEGIN_C_DECLS
      * is displayed.  If ignore_unknown is true, the error message is
      * not displayed.
      *
-     * Error messages are always displayed (to stderr, and
-     * OPAL_ERR_SILENT is returned) if a token was encountered that
-     * required N parameters, but <N parameters were found (e.g., "cmd
-     * --param foo", but --param was registered to require 2 option
-     * tokens).
+     * Error messages are always displayed regardless of the value
+     * of ignore_unknown (to stderr, and OPAL_ERR_SILENT is
+     * returned) if:
+     *
+     * 1. A token was encountered that required N parameters, but <N
+     * parameters were found (e.g., "cmd --param foo", but --param was
+     * registered to require 2 option tokens).
+     *
+     * 2. An unknown token beginning with "-" is encountered.  For
+     * example, if "--fo" is specified, and no "fo" option is
+     * registered (e.g., perhaps the user meant to type "--foo"), an
+     * error message is always printed, UNLESS this unknown token
+     * happens after a "--" token (see below).  
      *
      * The contents of argc and argv are not changed during parsing.
      * argv[0] is assumed to be the executable name, and is ignored during
@@ -400,8 +408,23 @@ BEGIN_C_DECLS
      * third parameter to the first instance of "foo", and "other" will be
      * an unrecognized option.
      *
-     * Invoking this function multiple times on different sets of argv
-     * tokens is safe, but will erase any previous parsing results.
+     * Note that -- can be used to allow unknown tokens that begin
+     * with "-".  For example, if a user wants to mpirun an executable
+     * named "-my-mpi-program", the "usual" way:
+     *
+     *   mpirun -my-mpi-program
+     *
+     * will cause an error, because mpirun won't find single-letter
+     * options registered for some/all of those letters.  But two
+     * workarounds are possible:
+     *
+     *   mpirun -- -my-mpi-program
+     * or
+     *   mpirun ./-my-mpi-program
+     *
+     * Finally, note that invoking this function multiple times on
+     * different sets of argv tokens is safe, but will erase any
+     * previous parsing results.
      */
     OPAL_DECLSPEC int opal_cmd_line_parse(opal_cmd_line_t *cmd, 
                                           bool ignore_unknown,