RaisePredictiveEvent works.

Needs sequence numbers fixed.

Content.Client/GameObjects/Components/Mobs/CombatModeComponent.cs

@@ -11,6 +11,7 @@ using Robust.Shared.IoC;
 namespace Content.Client.GameObjects.Components.Mobs
 {
     [RegisterComponent]
+    [ComponentReference(typeof(SharedCombatModeComponent))]
     public sealed class CombatModeComponent : SharedCombatModeComponent
     {
 #pragma warning disable 649

Content.Client/GameObjects/EntitySystems/CombatModeSystem.cs

@@ -2,6 +2,7 @@ using Content.Client.GameObjects.Components.Mobs;
 using Content.Client.UserInterface;
 using Content.Client.Utility;
 using Content.Shared.GameObjects.Components.Mobs;
+using Content.Shared.GameObjects.EntitySystemMessages;
 using Content.Shared.GameObjects.EntitySystems;
 using Content.Shared.Input;
 using JetBrains.Annotations;
@@ -13,6 +14,7 @@ using Robust.Client.Interfaces.Graphics.Overlays;
 using Robust.Client.Interfaces.Input;
 using Robust.Client.Player;
 using Robust.Shared.Input;
+using Robust.Shared.Interfaces.Timing;
 using Robust.Shared.IoC;
 using Robust.Shared.Maths;
 using Robust.Shared.Players;
@@ -30,6 +32,7 @@ namespace Content.Client.GameObjects.EntitySystems
         [Dependency] private readonly IPlayerManager _playerManager;
         [Dependency] private readonly IInputManager _inputManager;
         [Dependency] private readonly IOverlayManager _overlayManager;
+        [Dependency] private readonly IGameTiming _gameTiming;
 #pragma warning restore 649
 
         private InputSystem _inputSystem;
@@ -47,21 +50,20 @@ namespace Content.Client.GameObjects.EntitySystems
             _inputSystem = EntitySystemManager.GetEntitySystem<InputSystem>();
             _inputSystem.BindMap.BindFunction(ContentKeyFunctions.UseOrAttack, new InputHandler(this));
             _inputSystem.BindMap.BindFunction(ContentKeyFunctions.ToggleCombatMode,
-                InputCmdHandler.FromDelegate(CombatModeToggled, handle: false));
+                InputCmdHandler.FromDelegate(CombatModeToggled));
             _overlayManager.AddOverlay(new CombatModeOverlay(this));
         }
 
         private void CombatModeToggled(ICommonSession session)
         {
-            var player = IoCManager.Resolve<IPlayerManager>().LocalPlayer;
-
-            if (player.ControlledEntity == null ||
-                !player.ControlledEntity.TryGetComponent(out CombatModeComponent combatModeComponent))
+            if (_gameTiming.IsFirstTimePredicted)
             {
-                return;
-            }
+                EntityManager.RaisePredictiveEvent(
+                    new CombatModeSystemMessages.SetCombatModeActiveMessage(!IsInCombatMode()));
 
-            combatModeComponent.IsInCombatMode = !combatModeComponent.IsInCombatMode;
+                // Just in case.
+                UseOrAttackIsDown = false;
+            }
         }
 
         public override void Shutdown()
@@ -84,12 +86,12 @@ namespace Content.Client.GameObjects.EntitySystems
 
         private void OnTargetingZoneChanged(TargetingZone obj)
         {
-            EntityManager.RaisePredictiveEvent(new SetTargetZoneMessage(obj));
+            EntityManager.RaisePredictiveEvent(new CombatModeSystemMessages.SetTargetZoneMessage(obj));
         }
 
         private void OnCombatModeChanged(bool obj)
         {
-            EntityManager.RaisePredictiveEvent(new SetCombatModeActiveMessage(obj));
+            EntityManager.RaisePredictiveEvent(new CombatModeSystemMessages.SetCombatModeActiveMessage(obj));
 
             // Just in case.
             UseOrAttackIsDown = false;

Content.Server/GameObjects/Components/Mobs/CombatModeComponent.cs

@@ -9,6 +9,7 @@ namespace Content.Server.GameObjects.Components.Mobs
     ///     using *everything* as a weapon.
     /// </summary>
     [RegisterComponent]
+    [ComponentReference(typeof(SharedCombatModeComponent))]
     public sealed class CombatModeComponent : SharedCombatModeComponent
     {
     }

Content.Server/GameObjects/EntitySystems/CombatModeSystem.cs

@@ -5,34 +5,16 @@ using JetBrains.Annotations;
 using Robust.Server.GameObjects.EntitySystems;
 using Robust.Server.Interfaces.Player;
 using Robust.Shared.Input;
+using Robust.Shared.Interfaces.Random;
 using Robust.Shared.IoC;
+using Robust.Shared.Log;
 using Robust.Shared.Players;
+using Robust.Shared.Random;
 
 namespace Content.Server.GameObjects.EntitySystems
 {
     [UsedImplicitly]
     public sealed class CombatModeSystem : SharedCombatModeSystem
     {
-        public override void Initialize()
-        {
-            base.Initialize();
-
-            var inputSystem = EntitySystemManager.GetEntitySystem<InputSystem>();
-            inputSystem.BindMap.BindFunction(ContentKeyFunctions.ToggleCombatMode,
-                InputCmdHandler.FromDelegate(CombatModeToggled));
-        }
-
-        private static void CombatModeToggled(ICommonSession session)
-        {
-            var playerSession = (IPlayerSession) session;
-
-            if (playerSession.AttachedEntity == null ||
-                !playerSession.AttachedEntity.TryGetComponent(out CombatModeComponent combatModeComponent))
-            {
-                return;
-            }
-
-            combatModeComponent.IsInCombatMode = !combatModeComponent.IsInCombatMode;
-        }
     }
 }

Content.Shared/GameObjects/EntitySystems/SharedCombatModeSystem.cs

@@ -0,0 +1,40 @@
+using Content.Shared.GameObjects.Components.Mobs;
+using Content.Shared.GameObjects.EntitySystemMessages;
+using Content.Shared.Interfaces;
+using Robust.Shared.GameObjects;
+using Robust.Shared.GameObjects.Systems;
+using Robust.Shared.Interfaces.Random;
+using Robust.Shared.IoC;
+using Robust.Shared.Random;
+using Logger = Robust.Shared.Log.Logger;
+
+namespace Content.Shared.GameObjects.EntitySystems
+{
+    public abstract class SharedCombatModeSystem : EntitySystem
+    {
+        public override void Initialize()
+        {
+            base.Initialize();
+
+            SubscribeNetworkEvent<CombatModeSystemMessages.SetCombatModeActiveMessage>(CombatModeActiveHandler);
+            SubscribeLocalEvent<CombatModeSystemMessages.SetCombatModeActiveMessage>(CombatModeActiveHandler);
+        }
+
+        private void CombatModeActiveHandler(CombatModeSystemMessages.SetCombatModeActiveMessage ev, EntitySessionEventArgs eventArgs)
+        {
+            var entity = eventArgs.SenderSession.AttachedEntity;
+
+            if (!entity.TryGetComponent(out SharedCombatModeComponent combatModeComponent))
+            {
+                return;
+            }
+
+            if (IoCManager.Resolve<IModuleManager>().IsServerModule && IoCManager.Resolve<IRobustRandom>().Prob(0.5f))
+            {
+                Logger.Info("Mispredict!");
+                return;
+            }
+            combatModeComponent.IsInCombatMode = ev.Active;
+        }
+    }
+}