Index: docs/desync.txt
===================================================================
--- docs/desync.txt     (revision 26785)
+++ docs/desync.txt     (working copy)
@@ -194,6 +194,7 @@
    - Get the same version of OpenTTD as the original server was running.
    - Uncomment/enable the define 'DEBUG_DUMP_COMMANDS' in
      'src/network/network_func.h'.
+     (DEBUG_FAILED_DUMP_COMMANDS is explained later)
    - Put the 'commands-out.log' into the root save folder, and rename
       it to 'commands.log'.
    - Run 'openttd -D -d desync=3 -g startsavegame.sav'.
@@ -217,6 +218,11 @@
      the last dmp_cmds that reproduces the replay and the first one
      that fails.
 
+  If the replay does not succeed without mismatch, you can check the logs
+  whether there were failed commands. Then you may try to replay with
+  DEBUG_FAILED_DUMP_COMMANDS enabled. If the replay then fails, the
+  command test-run of the failed command modified the game state.
+
   If you have the original 'dmp_cmds_*.sav', you can also compare those
   savegames with your own ones from the replay. You can also comment/disable
   the 'NOT_REACHED' mentioned above, to get another 'dmp_cmds_*.sav' from
Index: src/network/network_func.h
===================================================================
--- src/network/network_func.h  (revision 26785)
+++ src/network/network_func.h  (working copy)
@@ -17,6 +17,7 @@
  * See docs/desync.txt for details.
  */
 // #define DEBUG_DUMP_COMMANDS
+// #define DEBUG_FAILED_DUMP_COMMANDS
 
 #include "core/address.h"
 #include "network_type.h"
Index: src/network/network.cpp
===================================================================
--- src/network/network.cpp     (revision 26786)
+++ src/network/network.cpp     (working copy)
@@ -929,7 +929,11 @@
                                p += 2;
                        }
 
-                       if (strncmp(p, "cmd: ", 5) == 0 || strncmp(p, "cmdf: ", 6) == 0) {
+                       if (strncmp(p, "cmd: ", 5) == 0
+#ifdef DEBUG_FAILED_DUMP_COMMANDS
+                               || strncmp(p, "cmdf: ", 6) == 0
+#endif
+                               ) {
                                p += 5;
                                if (*p == ' ') p++;
                                cp = CallocT<CommandPacket>(1);
@@ -958,6 +962,10 @@
                        } else if (strncmp(p, "msg: ", 5) == 0 || strncmp(p, "client: ", 8) == 0 ||
                                                strncmp(p, "load: ", 6) == 0 || strncmp(p, "save: ", 6) == 0) {
                                /* A message that is not very important to the log playback, but part of the log. */
+#ifndef DEBUG_FAILED_DUMP_COMMANDS
+                       } else if (strncmp(p, "cmdf: ", 6) == 0) {
+                               DEBUG(net, 0, "Skipping replay of failed command: %s", p + 6);
+#endif
                        } else {
                                /* Can't parse a line; what's wrong here? */
                                DEBUG(net, 0, "trying to parse: %s", p);